Promoting effect of CeO2 on the catalytic activity of Ba-Y2O3for direct decomposition of NO

Article abstract of DOI:10.1246/bcsj.20140230

The effect of CeO₂ additive on the catalytic performance of Ba-Y₂O₃ prepared by coprecipitaion for the direct decomposition of NO was investigated. Although Ba-Y₂O₃ effectively catalyzed NO decomposition, its activity was clearly increased by addition of CeO₂. The optimum CeO₂ content was 10 mol%. CO₂-TPD measurement revealed that the addition of CeO₂ into Ba-Y₂O₃ caused an increase in the CO₂ desorption peak in the temperature range of 473 and 723K derived from highly dispersed Ba species. The predominant role of CeO₂ additive was suspected to effectively create the highly dispersed Ba species as catalytically active sites. Kinetic studies of NO decomposition on Ba-CeO₂(10)-Y₂O₃ suggested that coexisting O₂ suppresses the NO decomposition reaction by competitive adsorption. Isotopic transient kinetic analysis suggested a reaction pathway in which the surface NO_x adspecies act as reaction intermediates for the formation of N₂ in NO decomposition over Ba-CeO₂-Y₂O₃. We concluded that CeO₂ additive does not directly participate in the NO decomposition reaction as catalytically active species.

Full text of DOI:10.1246/bcsj.20140230

Promoting Effect of CeO2 on the Catalytic Activity of Ba-Y2O3
for Direct Decomposition of NO
Yasuyuki Doi,1 Masaaki Haneda,*1 and Masakuni Ozawa2
1Advanced Ceramics Research Center, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu 507-0071
2EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603
E-mail: haneda.masaaki@nitech.ac.jp
Received: August 5, 2014; Accepted: September 4, 2014; Web Released: January 15, 2015
The effect of CeO2 additive on the catalytic performance of Ba-Y2O3 prepared by coprecipitaion for the direct
decomposition of NO was investigated. Although Ba-Y2O3 effectively catalyzed NO decomposition, its activity was
clearly increased by addition of CeO2. The optimum CeO2 content was 10 mol %. CO2-TPD measurement revealed that
the addition of CeO2 into Ba-Y2O3 caused an increase in the CO2 desorption peak in the temperature range of 473 and
723 K derived from highly dispersed Ba species. The predominant role of CeO2 additive was suspected to effectively
create the highly dispersed Ba species as catalytically active sites. Kinetic studies of NO decomposition on Ba-CeO2(10)-
Y2O3 suggested that coexisting O2 suppresses the NO decomposition reaction by competitive adsorption. Isotopic
transient kinetic analysis suggested a reaction pathway in which the surface NOx adspecies act as reaction intermediates
for the formation of N2 in NO decomposition over Ba-CeO2-Y2O3. We concluded that CeO2 additive does not directly
participate in the NO decomposition reaction as catalytically active species.
Since nitrogen oxides (NOx) are harmful to human health
and the global environment, the removal of NOx emitted from
combustion facilities is necessary. Use of catalysts is regarded
as an effective method for removing NOx from exhaust gases.
For example, three-way catalysts have already been practically
applied for the purification of exhaust gas emitted from
gasoline-powered vehicles. Selective catalytic reduction of
NOx using urea or ammonia is the leading technology for
meeting NOx emission from diesel exhaust. In these systems,
NOx are efficiently reduced by reacting with reductants such as
CO, H2, ammonia, and hydrocarbons.
On the other hand, direct decomposition of NO (2NO ¼
N2 + O2), which does not require use of reductants and is a
thermodynamically favorable reaction, is well known as the
most desirable reaction. Therefore, the development of effec-
tive catalysts for the direct decomposition of NO is a chal-
lenging subject of study for the solution of NOx problems.
Numerous studies have been performed so far to explore
catalysts showing activity for the direct decomposition of
NO, and have led to the discovery of many active catalysts
such as Cu-ZSM1 and perovskite oxides.2 However, there is
no catalyst that can be used under real conditions in the
presence of oxygen, water and carbon dioxide and at high space
velocity.
position reaction by Vannice et al.4 and Lunsford et al.5,6 One
of the authors also reported that the catalytic performance of
cobalt oxide can be effectively improved by addition of a small
amount of alkali or alkaline earth metals such as K and Ba.7-9
¹
They proposed a reaction mechanism in which NO2 species
adsorbed on alkali or alkaline earth metals react with adsorbed
¹
NO via the migration of NO2 species to the interface between
alkali or alkaline earth metals and cobalt oxide.10
We have recently investigated the catalytic performance of
Ba-doped rare earth oxides prepared by coprecipitation for the
direct decomposition of NO, and found that Ba species highly
dispersed on rare earth oxides with moderate strength basic
sites, such as Dy2O3, Sm2O3, and Y2O3, show relatively high
NO decomposition activity.11,12 The effectiveness of Ba species
and Y2O3 as catalytically active component and catalyst sup-
port, respectively, for the direct decomposition of NO has
also been reported by many researchers. For example, Ishihara
et al.13-15 reported Ba containing catalysts such as Ba/BaY2O4,
Ba/Y2O3, and Ba3Y3.4Sc0.6O9 as highly active catalysts.
Tsujimoto et al.16 studied the additive effect of BaO into C-
type cubic Y2O3-ZrO2 solid solution on the NO decomposition
performance, and found that Y2O3-ZrO2-BaO catalyst exhibits
relatively high NO decomposition activity even in the presence
of O2, H2O, or CO2.
Since oxygen-defect sites are believed to act as active sites
for the direct decomposition of NO,3 the strategy to develop
highly active catalysts investigated so far has been to design the
catalysts with specific structure promoting oxygen desorption.
On the other hand, alkaline earth oxides, which do not include
oxygen-deficient sites in the lattice, such as Sr/La2O3 and Ba/
MgO were reported to effectively catalyze the NO decom-
Among the catalysts investigated so far, Ba-Y2O3 seems to
be a highly active catalyst for the direct decomposition of NO.
It is of interest that the catalytic performance of Ba-Y2O3
continuously increased with the reaction time.11 This was
explained by the creation of highly dispersed Ba sites along
with the decomposition of BaCO3 as catalytically inactive
species. We also revealed that the formation of BaCO3 is
Bull. Chem. Soc. Jpn. 2015, 88, 117–123 | doi:10.1246/bcsj.20140230
© 2015 The Chemical Society of Japan | 117

favorable when Ba species was supported on rare earth oxides
with strong basic sites such as La2O3.12 In other words, the
selective formation of highly dispersed Ba species can be
expected by controlling the surface basicity of Y2O3, leading to
an increase in the NO decomposition activity. In this study, we
have investigated the effect of CeO2, which possesses weak
basicity, on the catalytic activity of Ba-Y2O3 for the direct
decomposition of NO. The purpose of this work is, accord-
ingly, to specify the importance of controlling the surface
basicity of support oxide for the creation of catalytically active
Ba species.
measurement was carried out up to 950 K at a heating rate of
¹1
¹1
10 K min in flowing He at a flow rate of 30 cm3 min
.
A quadrupole mass spectrometer (M-201QA-TDM, Canon
Anelva) was used to analyze the desorbed CO2.
The direct decomposition of NO was carried out in a fixed-
bed continuous flow reactor. The reaction gas composed of
1000 ppm NO with He as the balance gas was fed to a 0.5 g
catalyst that had been pretreated in situ in a flow of He at
¹1
¹3
1173 K for 8 h at a rate of 30 cm3 min (W/F = 1.0 g s cm
)
unless otherwise noted. The reaction temperature was decreas-
ed from 1173 to 873 K in steps of 50 K, and the steady-state
catalytic activity was measured at each temperature. The
effluent gas was analyzed by gas chromatography (Shimadzu
GC-8A) using a Molecular Sieve 5A column (for analysis of
O2 and N2) and a Porapak Q column (for analysis of N2O). A
chemiluminescence NOx analyzer (Shimadzu NOA-7000) was
used to check the stability of the catalytic activity.
The reaction rate of N2 formation was also measured under
nearly differential conditions giving NO conversion in the
range of 10-30% by varying the catalytic weight from 0.1 to
0.5 g. The concentrations of the reactants were varied in the
range of 500-3000 ppm for NO and 0-5% for O2 to determine
the kinetic parameters at 1073 K.
Isotopic transient kinetics analysis was performed by
switching the flowing gas from 1000 ppm 14NO to 1000 pppm
15NO diluted in He at 1173 K for a catalyst sample of 0.5 g. The
effluent gas from the reactor was continuously monitored by a
quadrupole mass spectrometer (PFEFFER OminiStar) for all
the isotopic molecules of NO (at m/e = 30 and 31), N2 (at 28,
29, and 30), and N2O (at 44, 45, and 46).
Experimental
CeO2-doped Ba-Y2O3 was prepared by coprecipitation using
(NH4)2CO3 as a precipitation agent, with an aqueous solution
of yttrium(III) nitrate, barium(II) nitrate, and cerium(III) nitrate.
The precipitate thus obtained was washed with distilled water,
followed by drying and calcination at 873 K for 5 h in air.
The resulting catalyst powder was finally calcined at 1173 K
for 5 h in air. The loading of Ba was fixed at 8 mol %, which
was found to be the optimum value,11 while that of CeO2 was
changed from 1 to 20 mol %. The samples are expressed as
Ba-CeO2(x)-Y2O3, where x is the loading of CeO2. The Ba
and CeO2 loadings in the catalysts were estimated by induc-
tively coupled plasma atomic emission spectroscopy (ICP-
AES; ICPS-8100, Shimadzu) and documented in Table 1. Cu-
ZSM-5 and K/Co3O4 with the K/Co atomic ratio of 0.035 as
the reference samples were also prepared by the same manner
as described elsewhere.7
The BET surface area of the samples was measured using a
volumetric adsorption apparatus (Micromeritics, TriStar II
3020) by nitrogen adsorption at liquid nitrogen temperature.
X-ray diffraction (XRD) patterns were measured using a
Rigaku MiniFlex diffractometer with Cu Kα radiation at 30 kV
and 15 mA to characterize the crystal structure of Ba-CeO2-
Y2O3 samples. Scanning electron microscopy (SEM; JSM-
7000F, JEOL) with energy-dispersive X-ray spectrometry
(EDS) was used to investigate the distributions of Y, Ba, and
Ce in the Ba-CeO2-Y2O3 samples.
Results and Discussion
Physicochemical Properties of Ba-CeO2-Y2O3. Table 1
summarizes the BET surface area of Ba-CeO2-Y2O3 with
different CeO2 content. It appears that no significant difference
in the BET surface area was obtained irrespective of CeO2
content. Figure 1 shows XRD patterns of Ba-CeO2-Y2O3 with
different CeO2 content. Distinct XRD peaks indexed to the
cubic phase of Y2O3 were observed for all the samples. It
should be noted that a shift of XRD peaks due to Y2O3 to
lower angle was observed when CeO2 content was increased
(Figure 1C). As also given in Table 1, the lattice constant of
Y2O3 calculated for the XRD peak due to Y2O3 (100) at
2ª = 29.0° was increased with increasing CeO2 content, sug-
gesting the formation of a solid solution. Y3+ ions are known to
be coordinated by 6 oxide anions in Y2O3.17 According to
literature,18 the ionic radii of Y3+, Ce3+, and Ce4+ for 6 coor-
dination are 0.090, 0.101, and 0.087 nm, respectively. Taking
into account the fact that the lattice constant of Y2O3 was
increased with CeO2 content, Ce3+ ions are suspected to be
incorporated into Y2O3 lattice. It is also noteworthy that the
XRD peak assignable to BaCO3 gradually decreased with
increasing CeO2 content (Figure 1B), indicating that the disper-
sion state of Ba species was improved by addition of CeO2
into Y2O3.
Temperature-programmed desorption of CO2 (CO2-TPD)
was performed using an atmospheric flow system (BELCAT,
BEL Japan). Before CO2-TPD measurement, the sample was
pretreated in a flow of O2 at 1173 K for 2 h, and then cooled
down to 323 K in flowing He. CO2 adsorption was performed
by passing a gas mixture of 0.5% CO2/He through the sam-
ple bed at 323 K for 1 h. After the adsorption gas was purged
with He until no CO2 was detected in the effluent, TPD
Table 1. Physicochemical Properties of Ba-CeO2-Y2O3
Samples
Composition
/mol %
BET
Lattice constant
of Y2O3
Catalyst
surface area
¹1
/m2 g
/nm
Ba CeO2
Ba-Y2O3
Ba-CeO2(1)-Y2O3
Ba-CeO2(5)-Y2O3
Ba-CeO2(10)-Y2O3 7.1
Ba-CeO2(20)-Y2O3 7.2
6.7
6.8
6.9
0
16.6
14.0
15.4
14.5
16.9
1.060
1.061
1.063
1.065
1.067
1.0
4.7
9.6
In order to obtain further information on the dispersion state
of Ba species, elemental mapping at the microstructural level
by SEM-EDS was performed for Ba-Y2O3 and Ba-CeO2(10)-
Y2O3. As can be seen in Figure 2A, an aggregation of Ba
19
118 | Bull. Chem. Soc. Jpn. 2015, 88, 117–123 | doi:10.1246/bcsj.20140230
© 2015 The Chemical Society of Japan

Figure 1. XRD patterns of Ba-CeO2-Y2O3 with various CeO2 contents. (a) Ba-Y2O3, (b) Ba-CeO2(1)-Y2O3, (c) Ba-CeO2(5)-
Y2O3, (d) Ba-CeO2(10)-Y2O3, (e) Ba-CeO2(20)-Y2O3. ( ) for Y2O3, ( ) for BaCO3.
species was observed for Ba-Y2O3. On the other hand, the
formation of relatively well dispersed Ba species was observed
for Ba-CeO2(10)-Y2O3 (Figure 2B). In addition, Ba species
seems to be present on CeO2 sites, suggesting the creation of
Ba-CeO2 interaction. In accordance with the results of XRD
mentioned above, CeO2 additive can improve the dispersion
state of Ba species.
In our previous study, the formation of BaCO3 was revealed
to be favorable when Ba species was supported on rare earth
oxides with strong basic sites such as La2O3.12 Therefore, the
fact that CeO2 additive suppressed the formation of BaCO3
may be ascribed to the creation of weak basic sites on the
surface of Y2O3 by interacting with CeO2. In order to confirm
this assumption, the surface basicity of Y2O3 and CeO2(10)-
Y2O3 was evaluated by CO2-TPD measurement. As can be seen
in Figure 3a, Y2O3 gave three CO2 desorption peaks at around
391, 512, and 636 K. Although CeO2(10)-Y2O3 showed similar
CO2-TPD profile with Y2O3, the presence of CeO2 caused an
increase in the CO2 desorption at the temperatures below 575 K
and a decrease in the CO2 desorption in the high-temperature
region (Figure 3b). In accordance with our assumption, the
addition of CeO2 seems to be lowered the strength of sur-
face basic sites of Y2O3, leading to the implementation of the
dispersion state of Ba species.
Catalytic Performance of Ba-CeO2-Y2O3 for NO
Decomposition.
Figure 4 shows the catalytic activity of
Ba-CeO2-Y2O3 catalyst with different CeO2 content for the
direct decomposition of NO. A good correlation between N2
yield and NO conversion was observed (results not shown). O2
was always formed at a steady state with an O2/N2 ratio
of approximately unity. The formation of N2O was hardly
observed in the entire temperature range. As can be seen in
Figure 4, the NO decomposition activity of Ba-Y2O3 was
improved by addition of CeO2. The activity of Ba-CeO2-Y2O3
increased with increasing CeO2 content, and reached a maxi-
Figure 2. SEM-EDS images of (A) Ba-Y2O3 and (B) Ba-
CeO2(10)-Y2O3.
Bull. Chem. Soc. Jpn. 2015, 88, 117–123 | doi:10.1246/bcsj.20140230
© 2015 The Chemical Society of Japan | 119

Figure 3. CO2-TPD profiles of (a) Y2O3 and (b) CeO2(10)-
Y2O3.
Figure 5. CO2-TPD profiles of Ba-CeO2-Y2O3 with vari-
ous CeO2 contents. (a) Ba-Y2O3, (b) Ba-CeO2(1)-Y2O3,
(c) Ba-CeO2(5)-Y2O3, (d) Ba-CeO2(10)-Y2O3, (e) Ba-
CeO2(20)-Y2O3.
Figure 4. Activity of Ba-CeO2-Y2O3 with various CeO2
contents for NO decomposition. ( ) Y2O3, ( ) CeO2(10)-
Y2O3, ( ) Ba-Y2O3, ( ) Ba-CeO2(1)-Y2O3, ( ) Ba-
CeO2(5)-Y2O3, ( ) Ba-CeO2(10)-Y2O3, ( ) Ba-
CeO2(20)-Y2O3. Conditions: NO: 1000 ppm, gas flow
¹3
rate: 30 cm3 min¹1, W/F = 1.0 g s cm
.
Figure 6. Change in the amount of CO2 desorption ( ) and
the rate of N2 formation at 1073 K ( ) over Ba-CeO2-
Y2O3 as a function of CeO2 content.
mum at the CeO2 content of 10 mol %. Since Y2O3 and
CeO2(10)-Y2O3 showed very low NO decomposition activity,
Ba species must be the catalytically active component.
858 K, indicating the creation of strong basic sites derived from
highly dispersed Ba species. When 1 mol % CeO2 was added to
Ba-Y2O3, the CO2-desorption peak at 858 K was significantly
shifted to lower temperature. New CO2-desorption peak at
624 K was observed. It is of interest that the CO2 desorption
temperature gradually decreased with increasing CeO2 content.
Taking into account the consideration that CO2 species adsorb-
ed on nanosized Ba sites is easily desorbed at lower tem-
perature, this result clearly indicates that CeO2 additive can
improve the dispersion states of Ba species. This is in agree-
ment with the results of XRD and SEM-EDS analyses.
We have recently studied the catalytic performance of Ba-
doped rare earth oxide catalysts for NO decomposition, and
revealed a good relationship between the N2 formation rate and
the number of Ba species highly dispersed on the surface of
rare earth oxide.12 The number of highly dispersed Ba species
was calculated from the amount of CO2 desorption peak
appearing in the temperature range of 573-873 K in the CO2-
TPD profile. In the present study, CO2-TPD profiles of Ba-
CeO2-Y2O3 with different CeO2 content were measured to
evaluate the number of highly dispersed Ba species.
Figure 5 shows the CO2-TPD profiles of Ba-CeO2-Y2O3
with different CeO2 content. From the comparison with CO2-
TPD profile of Y2O3 given in Figure 3a, the addition of Ba was
found to cause a disappearance of two desorption peaks at 512
and 636 K as well as an appearance of new desorption peak at
To gain information on the participation of highly dispersed
Ba species as catalytically active sites, the relationship between
the amount of desorbed CO2 in the temperature range of 473-
723 K and the rate of N2 formation at 1073 K was examined.
Figure 6 shows the change in the amount of desorbed CO2, as
120 | Bull. Chem. Soc. Jpn. 2015, 88, 117–123 | doi:10.1246/bcsj.20140230
© 2015 The Chemical Society of Japan

Figure 7. N2 formation rate over Ba-CeO2(10)-Y2O3 as a function of (A) O2 (0.5-5%) and (B) NO (500-3000 ppm) in the absence
¹1
and presence of 1% O2 at 1073 K. Conditions: NO: 500-3000 ppm, O2: 0.5-5%, gas flow rate: 30 cm3 min
.
well as the N2 formation rate, as a function of CeO2 content.
It appears that both the amount of desorbed CO2 and the N2
formation rate reached the maximum at the CeO2 content of
10 mol %, indicating that the number of highly dispersed Ba
species is responsible for high NO decomposition activity of
Ba-CeO2-Y2O3. Since Y2O3 and CeO2(10)-Y2O3 showed very
low NO decomposition activity (Figure 3), the predominant
role of CeO2 additive is to effectively create the highly
dispersed Ba species as catalytically active sites.
Kinetic Studies of NO Decomposition over Ba-CeO2-
Y2O3.
Since Ba-CeO2(10)-Y2O3 showed the highest NO
decomposition activity, as described above, we performed
kinetic studies to gain information on the reaction mechanism.
First, the effect of O2 concentration on the activity of Ba-
CeO2(10)-Y2O3 was measured. Figure 7A shows ln-ln plots of
N2 formation rate at 1173 K against the concentrations of O2.
Obviously, the N2 formation rate decreased with increasing O2
concentration. The reaction order with respect to O2 was ¹0.33.
In accordance with literature,3,17,19-21 this result indicates the
inhibitor action of O2 due to competitive adsorption between
NO and O2 onto the active sites. However, the negative effect
by coexisting O2 seems to be not serious in this case, compared
with conventional perovskite oxides such as La0.8Sro.2CoO3,3
La0.7Ba0.3Mn0.8In0.2O3,20 and Ba0.8La0.2Mn0.8Mg0.2321 on which
the reaction order with respect to O2 was ¹0.81, ¹0.53, and
¹0.81, respectively.
Figure 7B shows ln-ln plots of N2 formation rate over Ba-
CeO2(10)-Y2O3 at 1173 K as a function of NO concentration in
the absence and presence of 0.1% O2. The N2 formation rate
was found to monotonically increase with an increase in NO
concentration. The reaction order with respect to NO was 1.22
in the presence of O2 and 1.00 in its absence, suggesting that
NO decomposition proceeds via similar reaction pathway
irrespective of coexisting O2.
Figure 8. Product responses following the replacement of
14NO/He with 15NO/He in the reaction stream on Ba-
CeO2(10)-Y2O3 at 1173 K. ( ) 14N2, ( ) 14N15N, ( ) 15N2,
( ) 14NO, ( ) 15NO. Conditions: NO: 1000 ppm, gas flow
rate: 30 cm3 min¹1, catalyst weight: 0.5 g.
observation of adsorbed NOx species by in situ FT-IR spec-
troscopy and isotopic transient kinetic analysis, we proposed a
reaction mechanism in which the reaction is initiated by NO
¹
adsorption onto alkali metals to form NO2 species, which
migrates to the interface between the alkali metals and Co3O4,
and then react with the adsorbed NO species to form N2.
Recently, we have measured FT-IR spectra of NOx species
adsorbed on Ba-doped rare earth oxide under the conditions in
¹
flowing NO/He at 873 K, and observed the formation of NO2
species irrespective of rare earth oxide.12 Therefore, it can be
expected that NO decomposition reaction over Ba-CeO2-Y2O3
proceeds via similar reaction mechanism proposed for alkali
metal-doped Co3O4 catalyst.
We have so far measured the kinetic parameters of NO
decomposition over alkali metal-doped Co3O4 catalysts and
reported similar kinetic parameters obtained in this study.
Namely, the reaction order with respect to O2 and NO was
¹0.37 and 1.29 for K/Co3O4 catalyst, respectively.10 From the
In order to confirm the possibility mentioned above, isotopic
transient kinetic analysis was made on Ba-CeO2(10)-Y2O3.
Figure 8 shows the isotopic product responses following the
replacement of 14NO/He with 15NO/He in the reaction stream
at 1173 K. When 14NO was introduced to the reaction gas, 14N2
Bull. Chem. Soc. Jpn. 2015, 88, 117–123 | doi:10.1246/bcsj.20140230
© 2015 The Chemical Society of Japan | 121

extent of activity depression by O2 seems to be different
depending on the catalyst. The activity depression of Cu-ZSM-
5 was the most prominent, while that of Ba-CeO2-Y2O3 was
comparable to that of K/Co3O4. It is noteworthy that the
maximum NO conversion on Ba-CeO2-Y2O3 was higher than
those over Cu-ZSM-5 and K/Co3O4.
Conclusion
The catalytic activity of Ba-Y2O3 for the direct decom-
position of NO was effectively improved by addition of CeO2.
The optimum CeO2 content was found to be 10 mol %. CO2-
TPD measurements revealed that the addition of CeO2 into Ba-
Y2O3 caused a significant shift of CO2 desorption peak at
858 K ascribed to the desorption of CO2 adsorbed on highly
dispersed Ba species to lower temperatures around 588-624 K.
Since the relatively good relationship between the amount of
desorbed CO2 in the temperature range of 473-723 K and the
rate of N2 formation at 1073 K was observed, the number of
highly dispersed Ba species is responsible for high NO decom-
position activity of Ba-CeO2-Y2O3. From the kinetic studies
of NO decomposition on Ba-CeO2(10)-Y2O3, the reaction
order with respect to O2 and NO was found to be ¹0.33 and
1.00-1.22, respectively, suggesting the inhibitor action of O2.
Isotopic product responses following the 14NO ¼ 15NO switch
revealed the participation of surface NOx adspecies being
reaction intermediates in NO decomposition. The role of
CeO2 additive is not to directly participate in the NO decom-
position reaction as catalytically active sites but to effectively
create the highly dispersed Ba species as catalytically active
sites.
Figure 9. Catalytic activity of Ba-CeO2(10)-Y2O3 ( , ).
Cu-ZSM-5 ( , ) and K/Co3O4 (K/Co = 0.035) ( ,
)
for NO decomposition in the absence and presence of 1%
O2. Conditions: NO: 1000 ppm, O2: 0 or 1%, gas flow rate:
30 cm3 min¹1, W/F = 0.5 g s cm¹3. Open symbols ( ,
,
) indicate the results obtained in the absence of O2, and
solid symbols ( , ) in its presence.
,
and O2 (not shown) were detected as products. In accordance
with the results obtained under the steady-state conditions, no
formation of 14N2O was observed. When the reaction gas was
switched from 14NO/He to 15NO/He, a rapid decrease and
increase in the formation of 14N2 and 15N2 were observed,
respectively. It should be noted that the evolution of 14N15N
was simultaneously identified, indicating that the surface NOx
adspecies act as reaction intermediates in the formation of N2 in
NO decomposition. The dynamic behavior of 14N15N formation
on Ba-CeO2(10)-Y2O3 is in good agreement with that on alkali
metal-doped Co3O4 and Ba-doped rare earth oxide catalysts
reported so far. Therefore, we can conclude that NO decom-
position reaction over Ba-CeO2-Y2O3 proceeds via similar
reaction mechanism proposed for alkali metal-doped Co3O4
and Ba-doped rare earth oxide catalysts. CeO2 additive does
not directly participate in the NO decomposition reaction as
catalytically active sites. The predominant role of CeO2 addi-
tive is to effectively create the highly dispersed Ba species as
catalytically active sites.
This study was supported by a Grant-in-Aid for Scientific
Research (No. 22350068) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
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