Activity enhancement of WO3-promoted Ir/SiO2 catalysts by high-temperature calcination for the selective reduction of NO with CO

Article abstract of DOI:10.1246/bcsj.82.1023

The catalytic activity of WO₃-promoted Ir/SiO₂ for the selective reduction of NO with CO in the presence of excess O₂ is strongly dependent on the catalyst calcination conditions in the preparation process. WO₃/

Full text of DOI:10.1246/bcsj.82.1023

©
2009 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 82, No. 8, 1023–1029 (2009) 1023
Activity Enhancement of WO -Promoted Ir/SiO Catalysts
3
2
by High-Temperature Calcination for the Selective
Reduction of NO with CO
1
2
2
1
Masaaki Haneda,* Naoya Aoki, Kouji Arimitsu, and Hideaki Hamada
1Research Center for New Fuels and Vehicle Technology, National Institute of Advanced Industrial Science
and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565
2Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510
Received March 2, 2009; E-mail: m.haneda@aist.go.jp
The catalytic activity of WO3-promoted Ir/SiO2 for the selective reduction of NO with CO in the presence of excess
O2 is strongly dependent on the catalyst calcination conditions in the preparation process. WO3/Ir/SiO2 catalyst prepared
by air calcination showed higher activity than that prepared by H2 reduction. The activity of WO3/Ir/SiO2 catalyst
prepared by air calcination was also found to increase with increasing calcination temperature up to 1073 K; however, a
further increase in the temperature to 1173 K caused a decrease in the NO reduction activity, although very high
selectivity was observed for this catalyst. The lack of a major difference in the apparent activation energy for NO
reduction suggests that NO reduction occurs at the same reaction sites, irrespective of calcination temperature. On the
other hand, the apparent activation energy for CO oxidation increased with increased calcination temperature, suggesting
that the reaction sites for CO oxidation change according to calcination temperature. Catalyst characterizations revealed
that iridium species interacting strongly with WO3 are preferentially created by high-temperature air calcination. A
combination of partially reduced iridium species (IrO2¹x) and WO3 (IrO2¹x­WO3) was concluded to act as catalytically
active sites of WO3/Ir/SiO2 for NO reduction.
The selective catalytic reduction of NO in an oxidizing
activity at a high space velocity (over 50000 h¹1). We also
extensively studied the selective reduction of NO with CO over
Ir/WO ­SiO catalyst and found that the catalytic activity of
atmosphere is a promising technology for reducing NOx
emissions from diesel and lean-burn engines. A great number
of studies have been made recently on the use of hydrocarbons
3
2
Ir/WO ­SiO catalyst strongly depends on the catalyst pre-
3
2
1
13
as a reductant, while until recently CO has not been regarded
treatment conditions. When Ir/WO /SiO was treated in
3 2
as an effective reductant. The use of CO as a reductant is, in
practice, desirable, since vehicle exhaust generally contains
CO. It is also known that CO can be relatively easily formed by
controlling engine operation.
Various materials have been examined so far for catalytic
activity for the selective reduction of NO with CO in the
flowing 6% H O/He at 873 K after reduction in H at 873 K, its
2 2
activity was significantly enhanced. On the other hand, the
activity of Ir/WO3/SiO2 oxidized at 873 K following reduction
was quite low. On the basis of structural characterizations of the
catalysts treated under different conditions, it was concluded
that Ir metal species interacting strongly with W oxide (denoted
as Ir­WO ) are created by H O treatment as active sites for NO
presence of O . To our knowledge, a paper by Tauster et al. is
2
x
2
the first to report the selective reduction of NO with CO, which
reduction with CO. This suggests that the dispersion of Ir and
W oxide and their interactions are key factors in the catalytic
activity of Ir/WO /SiO . In the present study, we investigated
2
takes place over Ir/Al O . Ogura et al. found that NO can
2
3
successfully be reduced to N2 with CO over Ir/silicalite
catalyst and that the catalytic activity is not influenced by
3
2
the influence of the state of Ir and W oxide on the catalytic
activity of WO -added Ir/SiO by using WO /Ir/SiO catalysts
3
coexisting SO . Our group also discovered that Ir/SiO is an
2
2
3
2
3
2
effective catalyst for NO reduction with CO in the presence of
prepared by changing the catalyst calcination conditions in the
preparation process.
4­6
O and SO . The most interesting feature of this catalyst is
2
2
the promotive effect of coexisting SO on NO reduction, the
2
role of which is considered to stabilize the catalytically active
Experimental
0
7
8
9,10
Ir site in oxidizing atmospheres. Ir/ZSM-5 and Ir/WO
3
Catalyst Preparation. Ir/SiO2 was prepared by impregnation
have also been reported to effectively catalyze NO reduction
with CO under lean conditions. Iridium thus seems to be the
only active metal species to catalyze this reaction effectively.
Recently, Nanba et al. reported that the addition of W into
2 ¹1
of SiO2 (Fuji Silysia Chemicals, Cariact G-10, 300 m g ) with an
aqueous solution of [Ir(NH3)6](OH)3 (Tanaka Kikinzoku Kogyo),
followed by drying at 383 K overnight and then calcination at
873 K for 6 h in air. The loading of Ir metal was fixed at 0.5 wt % in
this study. (NH ) W O ¢5H O and citric acid were dissolved in
Ir/SiO drastically enhances NO reduction activity, even in the
2
4 10 12 41
2
11,12
absence of SO2.
The Ir/WO ­SiO catalyst showed high
distilled water to which the Ir/SiO2 had been added. The loading
3
2
Published on the web August 7, 2009; doi:10.1246/bcsj.82.1023

1
024 Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009)
NO Reduction with CO over WO3/Ir/SiO2
of WO3 was fixed at 10 wt %. The solvent was evaporated at
63 K, and the resulting mixture was dried at 383 K overnight and
3
1
00
calcined at 873 K for 6 h in air to obtain WO3/Ir/SiO2 powder. The
reduced Ir/SiO2 was also prepared by reducing at 873 K for 6 h in
flowing 10% H2/N2 after impregnation with Ir precursor. The
addition of W to the reduced Ir/SiO2 was performed in the same
manner as described above. The resulting W-added Ir/SiO2 was
again reduced at 873 K for 6 h in flowing 10% H2/N2. The
catalysts calcined in air are abbreviated as Ir/SiO2(Ox) and WO3/
Ir/SiO2(Ox), while those reduced with H2/N2 are abbreviated as
Ir/SiO2(Rd) and WO3/Ir/SiO2(Rd).
As for the WO3/Ir/SiO2(Ox) catalysts, the WO3/Ir/SiO2 was
further calcined at 973, 1073, and 1173 K for 6 h in air. In this case,
the samples are expressed as WO3/Ir/SiO2(x), where x is the
catalyst calcination temperature.
8
6
0
0
40
20
0
5
00
600
700
800
900
Temperature / K
1
00
Catalytic Activity Measurement.
Catalytic activity was
evaluated using a fixed-bed continuous flow reactor. A reaction gas
mixture containing NO (500 ppm), CO (3000 ppm), O2 (5%), SO2
8
6
4
2
0
0
0
0
0
(
1 ppm), and H2O (6%) diluted in He as the balance gas was fed
through a catalyst (0.04 g), pretreated in situ in a flow of He
3
¹1
¹1
at 873 K for 2 h, at a rate of 90 cm min (SV = ca. 75000 h ).
The effluent gas was analyzed with the use of two online gas
chromatographs equipped with a Molecular Sieve 5A column (for
analyzing N2 and CO) and a Porapak Q column (for analyzing CO2
and N2O). The reaction temperature was decreased from 873 to
4
73 K in steps of 20­50 K, and the steady-state catalytic activity
500
600
700
800
900
was measured at each temperature. N2 was mainly formed as the
NO reduction products. The selectivity to N2 (N2/(N2 + N2O))
was more than 90% in most cases
Temperature / K
Figure 1. Activity of Ir/SiO2 catalysts calcined under
different conditions for NO reduction with CO in the
presence of O and SO . ( ) Ir/SiO (Ox) calcined in air at
Catalyst Characterization.
The crystal structure was
2
2
identified by XRD (Mac Science M18XHF ) measurements using
Cu K¡ radiation at 40 kVand 150 mA. The BET surface area of the
samples was determined using a volumetric adsorption apparatus
2
2
2
873 K, ( ) Ir/SiO (Rd) reduced in flowing 10% H /N at
873 K. Conditions: NO, 500 ppm; CO, 3000 ppm; O , 5%;
SO , 1 ppm; H O, 6%; and W/F = 0.0267 g s cm .
2 2
2
2
2
2
¹
3
(
Quantachrome, Nova-4200e) by N2 adsorption at 77 K. Elemental
analysis for Ir and W was conducted using inductively coupled
plasma (ICP) using a Shimadzu ICPS-1000IV.
Table 1. Physical Properties of Ir/SiO2 and WO3/Ir/SiO2
The amount of chemisorbed CO was measured using a pulse
method. The sample (100 mg) was first reduced with H2 at 673 K
for 1 h, then cooled to 323 K in flowing He. Several pulses
of CO were introduced to the sample until no more adsorption
was observed. Iridium dispersion was calculated by assuming a
Calcination BET surface area Ir dispersion
Catalyst
2
¹1
atmosphere
/m g
(Ir/CO)
Ir/SiO2(Rd)
Ir/SiO2(Ox)
WO3/Ir/SiO2(Rd)
WO3/Ir/SiO2(Ox)
H2
Air
H2
261
267
235
235
0.61
0.10
1
4
a)
stoichiometry of 1.0 CO/Ir.
n.a.
a)
Temperature-programmed reduction (TPR) measurements were
carried out to estimate the reducibility of the WO3/Ir/SiO2
catalysts. Each catalyst sample (100 mg) was oxidized with 20%
O2/N2 at 873 K for 1 h and then cooled to room temperature. The
gas flow was then switched to 10% H2/Ar and the temperature was
Air
n.a.
a) n.a.: not available.
area of and Ir dispersion in Ir/SiO (Ox) and Ir/SiO (Rd). No
2
2
¹1
raised to 973 K at a rate of 10 K min . The consumption of H2 was
monitored using a thermal conductivity detector (TCD). Reduction
of CuO to metallic copper was used to calibrate the TPR apparatus
for H2 consumption.
difference in the BET surface was observed for both catalysts,
while Ir dispersion was considerably higher for Ir/SiO2(Rd)
than for Ir/SiO (Ox). We have already reported that the
2
catalytic activity of Ir/SiO for the selective reduction of NO
2
1
5
with CO is closely dependent on Ir dispersion: the turnover
frequency (TOF) for NO reduction decreased with increasing Ir
dispersion. This effect of Ir dispersion is explained by the
stability of catalytically active Ir metal species: larger iridium
crystallites show more resistance to oxidation and are much
more easily reduced under reaction conditions, resulting in the
formation of stable iridium metal sites on which NO reduction
Results and Discussion
Influence of Calcination Atmosphere. Activity of Ir/
SiO Catalyst: Figure 1 shows the activity of Ir/SiO (Ox)
and Ir/SiO (Rd) for NO reduction with CO in the presence of
O2 and SO2. It is apparent that little NO reduction occurred on
Ir/SiO (Rd) that had been reduced at 873 K. On the other hand,
2
2
2
2
Ir/SiO (Ox) showed NO reduction activity in the temperature
occurs. Consequently, the low activity of Ir/SiO (Rd) can be
2
2
range of 523­623 K, with the maximum NO conversion as high
as 40% at 553 K. In Table 1 are summarized the BET surface
ascribed to the oxidation of Ir metal to IrO under the reaction
conditions due to its high Ir dispersion.
2

M. Haneda et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009) 1025
:
IrO
2
,
: WO
3
100
80
60
40
20
0
(a)
(b)
(c)
500
600
700
800
900
Temperature / K
20
25 30 35 40 45 50 55 60
θ / degree
100
2
80
60
40
20
0
Figure 3. XRD patterns of (a) WO3/Ir/SiO2(Ox) calcined
in air at 873 K, (b) WO3/Ir/SiO2(Rd) reduced in flowing
1
0% H2/N2 at 873 K, and (c) WO3/Ir/SiO2(Rd) after use
in the reaction.
the XRD pattern of WO3/Ir/SiO2(Rd) (Figure 3b), suggesting
that Ir and W species are highly dispersed in WO /Ir/
3
500
600
700
800
900
SiO (Rd). This result indicates that Ir particles were not
2
aggregated by the addition of WO in the present study. We
Temperature / K
3
consider that the inhibition of CO adsorption on Ir by WO is
3
Figure 2. Activity of WO3/Ir/SiO2 catalysts calcined under
different conditions for NO reduction with CO in the
presence of O2 and SO2. ( ) WO3/Ir/SiO2(Ox) calcined in
air at 873 K, ( ) WO3/Ir/SiO2(Rd) reduced in flowing
probably due to a strong interaction of Ir with WO . The details
3
of the interaction between Ir and WO will be discussed in the
3
later part of this paper.
Since the selective reduction of NO with CO was carried out
under oxidizing conditions, highly dispersed Ir and W species
in WO /Ir/SiO (Rd) are likely to be easily oxidized to IrO
2
10% H2/N2 at 873 K. The reaction conditions are the same
as for Figure 1.
3
2
and WO during the reaction, respectively. In fact, distinct but
3
Activity of WO /Ir/SiO Catalyst: Figure 2 shows the
broad XRD peaks due to IrO and WO were observed in the
3
2
2
3
activity of WO /Ir/SiO (Ox) and WO /Ir/SiO (Rd) for NO
XRD pattern of WO /Ir/SiO (Rd) after use in the reaction
3 2
3
2
3
2
reduction with CO in the presence of O and SO . In contrast to
(Figure 3c). The formation of IrO and WO would result in
2 3
2
2
the results of Ir/SiO shown in Figure 1, WO /Ir/SiO (Rd)
low NO reduction activity. On the other hand, WO /Ir/
2
3
2
3
showed NO reduction activity. However, the activity of WO3/
Ir/SiO (Ox) was higher than that of WO /Ir/SiO (Rd). The
SiO2(Ox) showed high NO reduction activity (Figure 2),
although the presence of IrO and WO was observed in the
2
3
2
2
3
same results were obtained for CO oxidation activity.
XRD pattern (Figure 3a). As described in the experimental
section, the catalyst was pretreated in flowing He at 873 K prior
to the reaction, leading to the formation of partially reduced
iridium oxide (IrO2¹x) on which NO reduction with CO takes
In order to estimate the Ir dispersion, CO adsorption
measurements were also performed. However, no CO adsorp-
tion on WO /Ir/SiO was observed. We have also observed in
3
2
1
8
another set of experiments that Ir/WO /SiO , which was
place. However, the IrO2¹x species thus formed would not be
3
2
prepared by impregnating Ir precursor onto WO /SiO , does
easily oxidized to IrO under the reaction conditions, since
larger iridium crystallites show more resistance to oxidation.
3
2
2
1
6
15
not adsorb CO. Therefore, the phenomenon that CO was not
adsorbed on WO /Ir/SiO is not due to covering of the Ir sites
In conclusion, the catalyst pretreatment conditions in the
preparation process strongly affect the Ir species that are
responsible for the NO reduction activity. The air calcination of
supported Ir catalysts is an important procedure for obtaining
stable catalytically active Ir species under these reaction
conditions.
3
2
by WO . The inhibition of CO adsorption on supported Ir
3
1
7
catalysts by WO was also reported by Lecarpentier et al.
3
They ascribed this phenomenon to the aggregation of Ir
particles from the results of toluene hydrogenation, which is a
structure-insensitive reaction, taking place over Ir metal sites.
In order to clarify the possibility of the aggregation of Ir
particles by WO3 addition, XRD patterns of WO3/Ir/SiO2(Ox)
and WO /Ir/SiO (Rd) were measured. As shown in Figure 3a,
Influence of Calcination Temperature for WO /Ir/
3
SiO2(Ox) Catalysts. Catalytic Activity of WO3/Ir/SiO2:
Figure 4 shows the catalytic activity of WO /Ir/SiO calcined
3
2
3
2
peaks due to IrO (ICDD 15-870) and monoclinic WO (ICDD
in air at different temperatures for NO reduction with CO in the
presence of O and SO . The catalytic activity strongly depends
2
3
4
3-1035) were observed in the XRD pattern of WO /Ir/
3
2
2
SiO (Ox). On the other hand, no distinct peaks were detected in
on the calcination temperature. NO conversions at temperatures
2

1
026 Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009)
NO Reduction with CO over WO3/Ir/SiO2
1
00
1
00
8
6
0
0
8
6
4
2
0
0
0
0
0
40
2
0
500
600
700
800
900
0
0
5
10
15
Time on stream / h
Figure 5. Time on stream conversion of NO to N2 + N2O
20
25
30
Temperature / K
100
(
(
) and CO to CO2 ( ), and selectivity to N2 (N2/
N2 + N2O)) ( ) over WO3/Ir/SiO2(1073) catalyst for NO
8
6
4
2
0
0
0
0
0
reduction with CO in the presence of O2 and SO2 at 553 K.
The reaction conditions are the same as for Figure 1.
100
8
6
0
0
500
600
700
800
900
Temperature / K
Figure 4. Activity of WO3/Ir/SiO2 catalysts calcined at
different temperatures in air for NO reduction with CO in
the presence of O2 and SO2. Calcination temperature: ( )
40
873, ( ) 973, ( ) 1073, and ( ) 1173 K. The reaction
conditions are the same as for Figure 1.
20
below 533 K were clearly enhanced when the calcination
temperature was elevated from 873 to 1073 K. No great
difference in the activity for NO reduction was observed
between WO /Ir/SiO (973) and WO /Ir/SiO (1073), which
0
500
600
700
800
900
3
2
3
2
Temperature / K
showed the highest NO conversion. A further increase in the
calcination temperature to 1173 K, however, caused depres-
sion of NO conversion. On the other hand, CO conversion
monotonically decreased on increasing the calcination temper-
ature to 1073 K at the reaction temperatures above 573 K, after
which a significant drop in CO conversion was observed for
WO /Ir/SiO (1173) in the entire temperature range. In order to
Figure 6. Selectivity for NO reduction on WO3/Ir/SiO2
catalysts calcined at different temperatures in air for NO
reduction with CO in the presence of O2 and SO2.
Calcination temperature: ( ) 873, ( ) 973, ( ) 1073, and
(
) 1173 K. The reaction conditions are the same as for
Figure 1.
3
2
get information on the practical performance of the most active
WO /Ir/SiO (1073), the time on stream activity was measured
SN ¼ ð4 ꢀ N2 yield ðppmÞÞ=ð2 ꢀ CO2 yield ðppmÞÞ ꢀ 100
2
3
2
at 553 K. As shown in Figure 5, no significant decrease in the
NO conversion to N + N O was observed during the reaction
ð3Þ
The selectivity for NO reduction over WO /Ir/SiO thus esti-
2
2
3
2
until 30 h. The selectivity for N formation was almost constant
at about 90%. CO oxidation activity was found to be clearly
increased with time on stream.
mated is shown in Figure 6. Clearly, selectivity was improved
as the catalyst was calcined at higher temperatures. This result
suggests that Ir species on which NO reduction selectively takes
place are predominantly present in the WO /Ir/SiO (1173).
2
Concerning the selective reduction of NO with CO in the
3
2
presence of O , there are two unit reactions: NO reduction with
To obtain more information on the reaction sites, the
apparent activation energies for N and CO formation over
2
CO (eq 1) and CO oxidation by O (eq 2):
2
2
2
WO3/Ir/SiO2 were evaluated from the Arrhenius plots.
Figure 7 shows the Arrhenius plots of the reaction rates for
NO reduction to N (r ) and CO oxidation to CO (rCO)
measured under nearly differential reaction conditions giving
NO conversions of less than 20% by varying the catalyst
NO þ CO ! 1=2N2 þ CO2
CO þ 1=2O2 ! CO2
ð1Þ
ð2Þ
2
NO
2
On the basis of eqs 1 and 2, the selectivity for NO reduction
(
SN ) can be defined as follows.
2

M. Haneda et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009) 1027
weight. In Table 2 are summarized the apparent activation
energies estimated from the slopes of the Arrhenius plots. It is
of interest that almost the same apparent activation energies for
As given in Table 3, the surface area of WO /Ir/SiO has
3
2
2
¹1
considerably decreased from 235 m g for WO /Ir/SiO (873)
3
2
2
¹1
to 41 m g for WO /Ir/SiO (1173). However, the depression
3
2
NO reduction to N (E
) were observed for all the WO /Ir/
of the activity of WO /Ir/SiO by increasing the calcination
3 2
2
A,NO
3
¹
1
SiO catalysts (153­161 kJ mol ), indicating that NO reduc-
temperature from 873 to 1173 K was not prominent (Figure 4).
It is also noteworthy that WO /Ir/SiO (1073) showed the
2
tion to N2 occurs at the same reaction sites, irrespective
of calcination temperature. On the other hand, the apparent
activation energies for CO oxidation to CO (E
3
2
highest activity for NO reduction, in spite of its low surface
area.
) varied
A,CO
2
¹1
according to calcination temperature: 122 kJ mol for WO3/
The crystallite structure of WO /Ir/SiO was analyzed by
3
2
¹
1
Ir/SiO (873), 148 kJ mol for WO /Ir/SiO (973) and WO /
XRD. Figure 8 shows the observed XRD patterns. Clear XRD
peaks assignable to monoclinic WO and IrO were detected
2
3
2
3
¹1
Ir/SiO (1073), and 170 kJ mol for WO /Ir/SiO (1173). This
2
3
2
3
2
result is in good agreement with the selectivity shown in
Figure 6. The number of reaction sites for CO oxidation by O2
seems to be decreased by high-temperature calcination, result-
ing in an improvement of the reaction selectivity.
in all the samples. No difference in the crystallite structure
of WO /Ir/SiO was observed corresponding to calcination
3
2
temperature. Their peak width decreased with increasing
calcination temperature, indicating crystal growth. The crys-
tallite size of IrO and WO calculated from the XRD peaks
Catalyst Characterization: Since selective reduction of
NO with CO does not take place over WO /SiO , iridium must
2
3
due to the (110) plane at 2ª = ca. 28.1° (ICDD 15-870) and the
(222) plane at at 2ª = ca. 41.9° (ICDD 43-1035), respectively,
using Scherrer’s equation. As summarized in Table 3, the
crystallite size of IrO and WO increased with increasing the
3
2
be the catalytically active component. To obtain information on
the reaction sites, catalyst characterizations were performed for
WO /Ir/SiO calcined at different temperatures.
3
2
2
3
calcination temperature. This is in agreement with the results
of BET surface area. These results suggest the presence of
important factors, other than the number of reaction sites,
which would be related to the crystallite size of IrO and WO .
Table 2. Apparent Activation Energies of NO Reduction to
N2 (EA,NO) and CO Oxidation to CO2 (EA,CO)
2
3
Calcination
temperature/K /kJ mol
873 157.9
73
073
173
EA,NO
EA,CO
/kJ mol
The reduction behavior of WO /Ir/SiO was monitored by
Catalyst
3
2
¹
1
¹1
TPR. Normally, the reducibility of noble metals is considerably
WO3/Ir/SiO2(Ox)
121.9
147.4
148.4
169.8
19,20
influenced by the metal­support interaction.
Figure 9
9
157.5
160.8
153.2
shows the TPR profiles of WO /Ir/SiO along with those of
3
2
1
Ir/SiO and WO /SiO as the reference samples. Ir/SiO and
2
3
2
2
1
WO /SiO gave reduction peaks at 508 and above 723 K,
3
2
-
-
6.0
8.0
(
a)
(b)
-
-
7.0
9.0
-
-
10.0
12.0
-
-
-
11.0
13.0
15.0
-14.0
0
.0018
0.0019
0.0020
0.0018
0.0019
0.0020
-1
-1
-1
-1
T
/ K
T
/ K
Figure 7. Arrhenius plots of (a) reaction rate for NO to N2 and (b) reaction rate for CO to CO2 over WO3/Ir/SiO2 catalysts calcined
at different temperatures in air for NO reduction with CO in the presence of O2 and SO2. Calcination temperature: ( ) 873, ( ) 973,
(
) 1073, and ( ) 1173 K.
Table 3. Physical Properties of WO3/Ir/SiO2 Calcined at Different Temperatures Determined
Calcination temperature
BET surface area
Crystallite size/nm
Composition/wt %
Catalyst
2
¹1
Wa)
/K
/m g
IrO2
WO3
Ir
WO3/Ir/SiO2(Ox)
873
73
073
173
235
205
114
41
21.8
22.1
27.3
32.4
14.1
14.3
15.1
19.0
0.36
0.32
0.30
0.15
10.2
10.4
9.96
9.70
9
1
1
a) Composition of W was calculated as WO3.

1
028 Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009)
NO Reduction with CO over WO3/Ir/SiO2
Table 4. Summary of H2-TPR Measurements
:
IrO2, : WO3
Amount of H2 consumption
Calcination
temperature
¹1
/¯mol-H2 g-cat
Catalyst
Peak around
08 K
Peaks in
573­773 K
/K
5
(
d)
Ir/SiO2(Ox)
WO3/Ir/SiO2(Ox)
873
873
973
073
38.1
36.1
26.3
21.9
6.4
n.a.a)
41.7
76.7
61.5
19.9
(
(
c)
b)
1
1173
a) n.a.: not available.
(
a)
which can be ascribed to the reduction of oxygen present at the
Ir­WO3 interface. The presence of a strong Ir­WO3 inter-
action is also suggested by this result.
1
8
20
25 30 35 40 45 50 55 60
As can be seen in Figure 9, the amount of H consumption
2
2
θ / degree
for the reduction of IrO and oxygen present at the Ir­WO
2
3
interface is different according to the calcination temperature.
The quantitative results were summarized in Table 4. Obvi-
ously, the amount of H consumption for the reduction of IrO
Figure 8. XRD patterns of WO3/Ir/SiO2 calcined at
(a) 873, (b) 973, (c) 1073, and (d) 1173 K.
2
2
at around 508 K decreased with increasing calcination temper-
ature. The decrease in the H2 consumption would be due to a
loss of iridium during high temperature calcination, which was
confirmed by elemental analysis given in Table 3. The amount
of H consumption for the peaks between 573 and 773 K was
2
5
08
5
also summarized in Table 4. It is apparent that higher H2
consumption was obtained for WO /Ir/SiO (973) and WO /
28
(
(
f)
3
2
3
Ir/SiO (1073), suggesting that these catalysts contain a large
number of Ir species interacting strongly with WO3.
2
e)
Reaction Sites of WO3/Ir/SiO2:
As can be seen in
Figure 4, the catalytic activity of WO /Ir/SiO for NO
reduction with CO strongly depended on the calcination
temperature. Nevertheless, similar apparent activation energies
3
2
(
(
(
d)
c)
b)
for NO reduction to N (E
Ir/SiO2 catalysts (153­161 kJ mol ) (Table 2). This clearly
) were obtained for all the WO /
3
2
A,NO
¹
1
indicates that the reaction site for NO reduction on WO /Ir/
3
SiO is the same.
We reported that the active sites of Ir/WO3/SiO2 for NO
2
(
a)
reduction with CO are Ir­WO (x = 2.92­3) species in which Ir
metal interacts strongly with W oxide. This means that the
x
3
00 400 500 600 700 800 900
Temperature / K
13
presence of reduced Ir sites is necessary for the NO reduction
activity. In the present study, however, WO /Ir/SiO catalyst
3
2
Figure 9. TPR profiles of (a) Ir/SiO2, (b) WO3/SiO2 and
WO3/Ir/SiO2 calcined at (c) 873, (d) 973, (e) 1073, and
was pretreated not in flowing H but in flowing He at 873 K,
2
under which conditions iridium oxide (IrO ) is not completely
(
f) 1173 K.
2
reduced to Ir metal. As described before, a partially reduced
iridium oxide (IrO2¹x) would be formed by He treatment at
873 K. Nevertheless, WO /Ir/SiO effectively catalyzed the
respectively, ascribed to the reduction of IrO2 and WO3.
Although these two peaks were also observed for WO /Ir/
3
3
2
SiO , the temperature of H2 consumption peaks changed
NO reduction with CO in the presence of O and SO . This
2 2
2
according to the calcination temperature. WO /Ir/SiO (873)
fact suggests that the IrO2¹x species thus formed acts as a
catalytically active site for NO reduction. In addition, the
presence of a strong interaction between Ir and WO3 was
indicated from the TPR profiles of WO /Ir/SiO (973) and
3
2
gave the reduction peak of IrO at 508 K (Figure 9c), which is
2
the same as that for Ir/SiO , whereas the peak shifted to higher
2
temperatures for the other WO /Ir/SiO samples (Figures 9d­
3
2
3
2
9
f). The shift of this peak to a higher temperature suggests a
WO /Ir/SiO (1073) (Figures 9d and 9e). The amount of H
3 2 2
strong interaction between Ir and W species, leading to the
consumption for the reduction of oxygen present at the Ir­WO3
interface was also found to be higher for the both catalysts than
for other catalysts (Table 4). Since these catalysts showed high
NO reduction activity (Figure 4), it is suggested that a partially
consideration that the Ir­WO interaction can be created by
3
high-temperature calcination. In the TPR profiles of WO /Ir/
3
SiO (873), WO /Ir/SiO (973), and WO /Ir/SiO (1073), broad
2
3
2
3
2
H consumption peaks were also detected at around 573­773 K,
2
reduced IrO2¹x species interacting strongly with WO , abbre-
3

M. Haneda et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 8 (2009) 1029
viated as IrO2¹x­WO , is the catalytically active site for NO
The catalytic activity of WO /Ir/SiO prepared by air
3
3
2
reduction.
calcination was also found to strongly depend on the
calcination temperature. NO conversion was clearly enhanced
by elevating the calcination temperature from 873 to 1073 K.
The reaction site for CO oxidation by O2 is probably
different from that for NO reduction. As can be seen in
Figure 6, WO /Ir/SiO (873) was the least selective catalyst for
The highest NO conversion was achieved on WO /Ir/
3
2
3
NO reduction. In the TPR profiles of WO /Ir/SiO (873), the
SiO (1073). Iridium species interacting strongly with WO
3
3
2
2
H2 consumption peak for IrO2 was at the same temperature as
that for Ir/SiO . The ratio of H consumption for the peaks
were found to be selectively created by high-temperature
calcination. The active site of WO /Ir/SiO for NO reduction
2
2
3
2
between 573 and 773 K to that for the peak at 508 K was found
to be 1.16, which is quite small compared with the other
catalysts (2.81­3.11), suggesting the presence of less iridium
species interacting strongly with WO in WO /Ir/SiO (873).
consisted of a combination of partially reduced IrO2¹x and
WO , i.e., IrO ­WO species, formed by He pretreatment at
3
2¹x
3
873 K. Oxidation of CO by O was believed to take place
2
preferentially over iridium species not interacting with WO3.
3
3
2
Taking into account the fact that the apparent activation energy
¹
1
for CO oxidation (EA,CO) on WO /Ir/SiO (873) (122 kJ mol )
A part of this work has been supported by the New Energy
and Industrial Technology Development Organization (NEDO)
under the sponsorship of Ministry of Economy, Trade and
Industry (METI) of Japan.
3
2
¹
1
was different from that on other catalysts (147­170 kJ mol ),
CO oxidation by O preferentially takes place over iridium
species not interacting with WO . Thus, the ratio of iridium
2
3
species with or without an interaction with WO determines the
3
activity of WO /Ir/SiO for NO reduction.
3
2
References
Since WO /Ir/SiO (1173) showed good selectivity for NO
3
2
1
2
3
H. Hamada, Catal. Today 1994, 22, 21.
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3
SiO (1173) are believed to interact strongly with WO . It is
2
3
noteworthy that the H2 consumption peak of IrO2 for WO3/Ir/
SiO (1173) is very weak compared with that for WO /Ir/
Lett. 2000, 146.
2
3
4
T. Yoshinari, K. Sato, M. Haneda, Y. Kintaichi, H. Hamada,
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2
Catal. Commun. 2001, 2, 155.
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5
T. Yoshinari, K. Sato, M. Haneda, Y. Kintaichi, H. Hamada,
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analysis given in Table 3. While the weight composition of
iridium in WO /Ir/SiO (873) (0.36 wt %) was slightly lower
3
2
6
than the expected value from the preparation conditions, the
weight composition of iridium in WO /Ir/SiO decreased on
3
2
7
increasing the calcination temperature, resulting in a sharp drop
to 0.15 wt % for WO /Ir/SiO (1173). A loss of iridium after
3
2
8
the calcination at 1173 K would be due to a vaporization of
iridium oxide during the calcination, because it is known that
IrO is converted to IrO under oxidizing conditions at the
9
M. Shimokawabe, N. Umeda, Chem. Lett. 2004, 33, 534.
2
3
10 H. Inomata, M. Shimokawabe, M. Arai, Appl. Catal., A
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A. Obuchi, Chem. Lett. 2006, 35, 450.
temperatures above 1173 K and IrO3 thus formed is easily
vaporized because of its high vapor pressure.21 In spite of such
a low loading of Ir, it is of interest that WO /Ir/SiO (1173)
3
2
1
2
T. Nanba, S. Shinohara, S. Masukawa, J. Uchisawa, A. Ohi,
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showed relatively high NO reduction activity (Figure 4), with a
maximum NO conversion of 60% at 573 K. It can be concluded
1
3
that iridium species strongly interacting with WO are the only
3
species encountered in the WO /Ir/SiO (1173).
3
2
1
4
Conclusion
1
5
WO /Ir/SiO catalyst prepared by air calcination showed
3
2
higher activity for the selective reduction of NO with CO than
that by H2 reduction. Although the formation of highly
dispersed Ir and W species in WO /Ir/SiO by H reduction
1
6
3
2
2
1
7
was suggested from the XRD pattern, the Ir and W species were
confirmed to be easily oxidized to IrO2 and WO3 during
the reaction, resulting in a low NO reduction activity. On the
1
1
8
9
M. Haneda, H. Hamada, Chem. Lett. 2008, 37, 830.
A. Trovarelli, Catal. Rev., Sci. Eng. 1996, 38, 439.
other hand, as for the WO /Ir/SiO catalyst prepared by air
3
2
20 H. C. Yao, M. Sieg, H. K. Plummer, J. Catal. 1979, 59,
365.
21 Iwanami Rikagakujiten, 4th ed., ed. by R. Kubo, S.
Nagkura, H. Inokuchi, H. Ezawa, Iwanami Shoten, Tokyo, 1987,
p. 492.
calcination, a partially reduced iridium oxide (IrO2¹x) formed
by He treatment at 873 K as a pretreatment for the reaction was
considered to be stable under the reaction conditions, leading to
high NO reduction activity.