Promotional effect of SO2 on the activity of Ir/SiO2 for NO reduction with CO under oxygen-rich conditions

Article abstract of DOI:10.1016/j.jcat.2004.10.022

The effect of coexisting SO₂ on the activity of silica-supported noble metal catalysts for the selective reduction of NO with CO in the presence of O₂ was investigated. Pt/SiO₂, Rh/SiO₂, and Pd/SiO₂ showed little catalytic activity for NO reduction, irrespective of coexisting SO₂. Although Ir/SiO₂ showed no NO reduction activity in the absence of SO₂, the presence of SO ₂ drastically promoted NO reduction. A comparison of the catalytic performance of Ir/SiO₂ and Ir/Al₂O₃ in the presence of SO₂ showed that Ir supported on SiO₂ is more active than Ir on Al₂O₃. SiO₂ was found to be a more effective support than Al₂O₃. The most outstanding feature of the reaction on the Ir/SiO₂ catalyst was that the coexistence of SO₂ and O₂ is essential for NO reduction to occur. The role of coexisting SO₂ was considered to be not only to stabilize but also to create Ir⁰ sites in an oxidizing atmosphere. FT-IR measurements suggested that a cis-type coordinated species of NO and CO on one iridium atom () was an intermediate for NO reduction by CO. Although the species completely disappeared with the addition of O₂ to the reaction gas, the presence of coexisting SO₂ caused a reappearance of the species. A reaction mechanism in which N₂ and N₂O are produced via the recombination of dissociated N atoms (N_(a) + N _(a) → N₂) and the formation of dimer (NO) ₂-type species (2NO → (NO)_2(a) → N₂O + O_(a)), respectively, is proposed.

Full text of DOI:10.1016/j.jcat.2004.10.022

Journal of Catalysis 229 (2005) 197–205
www.elsevier.com/locate/jcat
Promotional effect of SO2 on the activity of Ir/SiO2 for NO reduction
with CO under oxygen-rich conditions
Masaaki Haneda a,∗, Pusparatu b, Yoshiaki Kintaichi a, Isao Nakamura a, Motoi Sasaki a,
Tadahiro Fujitani a, Hideaki Hamada a
a
Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST),
AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
b
Department of Materials Science and Technology, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan
Received 16 September 2004; revised 25 October 2004; accepted 25 October 2004
Available online 8 December 2004
Abstract
The effect of coexisting SO on the activity of silica-supported noble metal catalysts for the selective reduction of NO with CO in the
2
presence of O was investigated. Pt/SiO , Rh/SiO , and Pd/SiO showed little catalytic activity for NO reduction, irrespective of coexisting
2
2
2
2
SO . Although Ir/SiO showed no NO reduction activity in the absence of SO , the presence of SO drastically promoted NO reduction.
2
2
2
2
A comparison of the catalytic performance of Ir/SiO and Ir/Al O in the presence of SO showed that Ir supported on SiO is more
2
2
3
2
2
active than Ir on Al O . SiO was found to be a more effective support than Al O . The most outstanding feature of the reaction on the
2
3
2
2 3
Ir/SiO catalyst was that the coexistence of SO and O is essential for NO reduction to occur. The role of coexisting SO was considered
2
2
2
2
0
to be not only to stabilize but also to create Ir sites in an oxidizing atmosphere. FT-IR measurements suggested that a cis-type coordinated
NO
CO
NO
CO
species of NO and CO on one iridium atom (Ir
) was an intermediate for NO reduction by CO. Although the Ir
species completely
NO
disappeared with the addition of O to the reaction gas, the presence of coexisting SO caused a reappearance of the Ir
species.
2
2
CO
A reaction mechanism in which N and N O are produced via the recombination of dissociated N atoms (N + N → N ) and the
2
2
(a)
(a)
2
formation of dimer (NO) -type species (2NO → (NO)
→ N O + O ), respectively, is proposed.
2
2(a)
2
(a)
2004 Elsevier Inc. All rights reserved.
Keywords: Selective reduction; Carbon monoxide; Nitrogen monoxide; Iridium catalyst; SO promoting effect
2
1. Introduction
drocarbons as reductants for the selective catalytic reduction
of NO (HC-SCR) [1–3]. Nevertheless, practical application
of HC-SCR is not easy, because its catalytic performance is
insufficient. In particular, catalyst deactivation and reaction
inhibition by coexisting SO2 and H2O are major problems to
be solved.
Hydrogen and CO were not regarded as effective reduc-
tants until recently. In 1997, Yokota et al. [4] reported the
activity of Pt/mordenite for NO reduction with H2 in the
presence of O2 around 423 K. They also found that the addi-
tion of Mo and Na widens the temperature window. Later,
several researchers studied the selective reduction of NO
with H2 over Pt- and Pd-based catalysts [5–8]. As for NO
reduction with CO, Ogura et al. [9] investigated the activ-
The selective reduction of NO in an oxidizing atmosphere
has recently received extensive attention, since it has poten-
tial as a practical strategy for removing NOx emitted from
diesel engines, lean-burn engines, and combustors. Although
NH3 is currently used as an efficient selective reductant for
flue gases from large-scale boilers, it cannot be used for
small-scale or mobile NOx sources. In this regard, a great
number of studies have been made recently on the use of hy-
*
Corresponding author. Fax: +81-298-61-4647.
E-mail address: m.haneda@aist.go.jp (M. Haneda).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.10.022
2004 Elsevier Inc. All rights reserved.

198
M. Haneda et al. / Journal of Catalysis 229 (2005) 197–205
ity of supported iridium catalysts and found that NO can be
successfully reduced to N2 with CO over Ir/silicalite catalyst
and that the catalytic activity is not influenced by coexisting
SO2. Wang et al. [10] reported that among Pt, Pd, Rh, and
Ir catalysts, Ir/ZSM-5 catalyst exhibited the highest activ-
ity for NO reduction by CO in the presence of excess O2.
The selective reduction of NO with CO was also reported to
take place catalytically over supported metal oxide catalysts
such as Cu/Al2O3 [11]. Comparison of the activity of vari-
ous supported metallic catalysts under the same conditions
revealed, however, that the activity of Cu/Al2O3 is not very
high, and that supported iridium catalyst is the most active
for NO reduction with CO [12].
We have recently discovered that Ir/SiO2 and Rh/SiO2
show marked catalytic activity for NO reduction with H2 in
the presence of O2 and SO2 [13,14]. The most significant
feature of this reaction is that the presence of SO2, which
normally poisons catalytic reactions, actually promotes NO
reduction in the presence of O2. This is quite a favorable
characteristic for the treatment of diesel exhaust. However,
no evidence has yet been obtained to explain this promot-
ing effect of SO2. In this paper, we report that CO, which is
a more practical reductant than H2, also serves as an effec-
tive reductant for NO reduction over Ir/SiO2 catalyst in the
presence of O2 and SO2. A mechanistic role for SO2 is also
proposed, based on in situ IR measurements.
bed at a rate of 90 cm3 min−1 (SV = ca. 75000 h−1). In some
experiments, the concentration of each component gas was
changed. The effluent gas was analyzed with the use of two
on-line gas chromatographs equipped, respectively, with a
Molecular Sieve 5A column (for the analysis of N2 and CO)
and a Porapak Q column (for the analysis of CO2 and N2O).
The reaction temperature was reduced from 873 to 473 K in
steps of 25–50 K, and the steady-state catalytic activity was
measured at each temperature.
2.3. Catalyst characterization
The amount of chemisorbed CO was measured with a
pulse method. The sample (0.05 g) was first reduced with
H2 at 673 K for 1 h, then cooled to room temperature in
flowing He. Several pulses of CO were introduced into the
sample until no more adsorption was observed.
The crystal structure was identified by XRD (Mac Sci-
ence M18XHF22) measurements with Cu-Kα radiation at
40 kV and 150 mA. TEM analysis was made with a Hitachi
H-9000NAR at an acceleration voltage of 200 kV.
2.4. FT-IR study
Diffuse reflectance FT-IR spectra were recorded with a
Nicolet Nexus 670 FT-IR spectrometer, which accumulated
64 scans at a resolution of 4 cm−1. Prior to each experiment,
12 mg of a catalyst placed in a diffuse-reflectance high-
temperature cell (Spectra Tech) fitted with CaF2 windows
was pretreated in situ by heating in flowing 10% H2–6%
H2O/He at 873 K and then purged with He for 1 h, followed
by cooling to the desired temperature. The background spec-
trum of the clean surface was measured for spectral correc-
tion. Surface species were observed after introduction of a
reaction gas containing one or more of the gas components
1000 ppm NO, 6000 ppm CO, 5% O2, or 20 ppm SO2 at a
2. Experimental
2.1. Catalyst preparation
Silica-supported noble metal (Pt, Rh, Pd, Ir) catalysts
were prepared by impregnation of SiO2 (Fuji Silysia Chem-
icals, Cariact G-10, 300 m2 g−1) with aqueous solutions of
[Pt(NH3)4](NO3)2 (aqueous solution, Pt content = 5.77%;
N.E. Chemcat), Rh(NO3)3 (Soekawa Chemicals),
[Pd(NH3)4](NO3)2 (N.E. Chemcat), or H2IrCl6 · 6H2O
(Soekawa Chemicals). For comparison, Ir/Al2O3 was also
prepared by impregnation of Al2O3 (Mizusawa Chemi-
cals, GB-45, 190 m2 g−1) with an aqueous solution of
H2IrCl6 · 6H2O. The impregnated catalyst precursors were
dried at 383 K and finally calcined at 873 K for 8 h in air.
The loading of noble metals was fixed at 5 wt%.
flow rate of 30 cm3 min−1
.
2.5. Isotopic transient kinetic analysis
Isotopic transient kinetic analysis was carried out by
switching the flowing gas from 15NO–CO–O2–SO2/He to
14NO–CO–O2–SO2/He at 523 K for a catalyst sample of
0.02 g. The effluent gas from the reactor was continuously
monitored by a quadrupole mass spectrometer (ANELVA
M-QA200TS) for all of the isotopic products of N2 (at
m/e = 28, 29 and 30) and N2O (at 44, 45, and 46).
2.2. Catalytic activity measurement
Catalytic activity was evaluated with a fixed-bed contin-
uous-flow reactor. A catalyst sample (0.04 g) was held in a
quartz tube (10 mm i.d.) by quartz wool packed into both
ends of the catalyst bed. Prior to each reaction, the catalyst
was pretreated in situ in a flow of 10% H2–6% H2O/He at
873 K for 1 h, unless otherwise specified.
3. Results and discussion
3.1. Activity of noble metal catalysts
The standard reaction gas, containing NO (1000 ppm),
CO (6000 ppm), O2 (5%), SO2 (20 ppm), and H2O (6%)
diluted in He as the balance gas, was fed through the catalyst
Table 1 summarizes the catalytic activity of the silica-
supported noble metal catalysts for NO reduction with

M. Haneda et al. / Journal of Catalysis 229 (2005) 197–205
199
Table 1
NO reduction with CO over supported noble metal (5 wt%) catalysts
Catalyst
O
SO
NO conversion to N (N O) (%)
CO conversion to CO (%)
2
2
2
2
2
(%) (ppm)
423 K
473 K
573 K
623 K
673 K
723 K
773 K
423 K 473 K 573 K 623 K 673 K 723 K 773 K
Pt/SiO
5
5
0
20
0 (3.2)
0 (0.5)
0 (1.2) 0 (1.3) 0 (0.3)
0 (1.0) 0 (0.7) 0 (0.4)
0 (0.2) 0 (0)
0 (0.6) 0 (0.3)
0 (0) 74
0 (0.2) 4.5
100
98
100
100
100
100
100
100
100
100
100
100
2
Rh/SiO
5
5
0
20
0 (0.3)
0 (0.7) 0 (0.2) 0 (0.2)
0 (0.6) 0 (1.1) 0 (1.7)
0 (0)
0 (1.3) 0 (0.6)
0 (0)
0 (0) 11
0 (0.2)
97
2.6
100
12
100
46
100
95
100
100
100
100
2
2
Pd/SiO
5
5
0
20
0 (2.3)
0 (0.2) 0 (0.2) 0 (0)
0 (0.7) 0 (1.2) 0 (0.7)
0 (0) 0 (0)
0 (0.5) 0 (0.2)
0 (0) 95
0 (0)
100
28
100
98
100
100
100
100
100
100
100
100
Ir/SiO
0
5
5
0
0
20
7.4 (2.2) 54 (3.6) 61 (2.0) 43 (1.5) 44 (0)
0 (0) 0 (0) 0 (0) 0.7 (0.2) 0 (0)
3.3 (0.9) 11 (2.4) 62 (6.0) 28 (3.2) 14 (1.5) 4.1 (0.8)
79 (0)
0 (0)
4.9
2.9
25
77
17
24
97
90
52
100
96
99
100
97
96
100
99
2
Ir/Al O
0
5
5
0
0
20
0.8 (0.9) 35 (4.6) 30 (2.8) 83 (4.8) 84 (1.2) 42 (0)
6.4
0.9
1.8
32
81
4.1
34
98
17
19
100
67
21
100
96
96
100
100
2
3
0 (0)
0 (0) 1.4 (0.2) 5.7 (0.4) 12 (0.7) 7.3 (0.6)
0 (0.6) 0 (0.6) 2.8 (1.1) 15 (1.9) 6.1 (1.1) 2.4 (0.3)
−3
Conditions: NO = 1000 ppm, CO = 6000 ppm, O = 0 or 5%, SO = 0 or 20 ppm, H O = 6%, W/F = 0.0267 g s cm
.
2
2
2
6000 ppm CO in the presence of 5% O2 either with or with-
out 20 ppm SO2. The reproducibility of the catalytic activity
data was fairly good in this study.
Table 2
Dispersion of iridium on catalysts
Catalyst
Dispersion
Amount of CO adsorption
−1
(CO/noble metal)
(mol g
)
Pt/SiO2, Rh/SiO2, and Pd/SiO2 showed little catalytic
activity for NO reduction to N2/N2O over the entire tem-
perature range, irrespective of coexisting SO2. The perfor-
mance of Ir/SiO2, on the other hand, was markedly differ-
ent. Although NO reduction did not take place in the ab-
sence of SO2, the presence of coexisting SO2 drastically
promoted NO reduction. At 623 K, for instance, the pres-
ence of 20 ppm SO2 increased NO conversion to N2/N2O
from 0 to 68%. In contrast to NO conversion, CO conver-
sion was decreased by the presence of SO2, suggesting that
coexisting SO2 improves the utilization of CO for NO reduc-
tion. In conclusion, iridium is the only active noble metal
that allows NO reduction with CO in the presence of O2
and SO2.
cat
−5
−5
Ir/SiO
2
0.09
0.05
2.26 × 10
1.34 × 10
Ir/Al O
2
3
To obtain information on the physical properties of sup-
ported Ir catalysts, the amount of CO adsorption was first
measured with the pulse method. As summarized in Table 2,
no major difference in the amount of CO adsorption, corre-
sponding to the number of Ir atoms exposed on the surface,
was observed for Ir/SiO2 and Ir/Al2O3, although the former
catalyst was more active than the latter one (Table 1). This
suggests that the dispersion of Ir does not significantly affect
the catalytic performance.
TEM observation was also performed for Ir/SiO2 and
Ir/Al2O3 to examine the structure of supported Ir parti-
cles. Fig. 1 shows the TEM images of Ir/SiO2 and Ir/Al2O3
treated with an H2–H2O/He flow at 873 K. From Fig. 1a-1,
it was found that Ir particles are well dispersed with a size
of 10–20 nm over the surface of the SiO2 support. The lat-
tice constant of Ir particles calculated from the TEM images
(Fig. 1a-2) indicated the formation of the metal phase of Ir
by the H2–H2O treatment at 873 K. This is in accordance
with the results of the XRD measurements described below.
On the other hand, the structure of Ir particles deposited on
Al2O3 seems to be different from the structure of those on
SiO2 (Fig. 1b). The Ir particles on Al2O3 clearly consist of
agglomerates of small particles with a size of 2–3 nm, and
their formation is likely to be responsible for the low NO
reduction activity of Ir/Al2O3. The rest of this paper deals
mainly with Ir/SiO2 catalyst, which showed the highest NO
reduction activity.
3.2. Comparison of Ir/SiO2 and Ir/Al2O3
Table 1 also shows the activity of Ir/Al2O3 catalyst. It
is noteworthy that the catalytic activity of Ir/Al2O3 was little
affected by the presence of SO2, although a slight downward
shift of the effective temperature window was observed.
Thus, SiO2 is a more effective support than Al2O3 for NO
reduction with CO in the presence of O2 and SO2. The dif-
ferent activities of Ir/SiO2 and Ir/Al2O3 may be attributable
to the difference in the amount of residual chloride ions on
SiO2 and Al2O3. However, Ir/Al2O3 prepared with Ir(NO3)4
(N.E. Chemcat) as a Cl-free iridium precursor showed a cat-
alytic performance (results not shown) similar to that pre-
pared with H2IrCl6 · 6H2O, suggesting that the influence of
residual chloride ions can be ruled out. This is probably due
to the fact that chloride ions are almost completely removed
by treatment with H2–H2O/He gas mixture before the reac-
tion.

200
M. Haneda et al. / Journal of Catalysis 229 (2005) 197–205
Fig. 1. TEM images of (a) 5% Ir/SiO and (b) 5% Ir/Al O treated in flowing 10% H –6% H O/He at 873 K.
2
2
3
2
2
Fig. 3. XRD patterns of Ir/SiO treated in flowing (a) 5% O /He and
2
2
0
(b) 10% H –6% H O/He at 873 K. (Q) for IrO , (!) for Ir .
2
2
2
Fig. 2. Effect of pretreatment conditions on the activity of Ir/SiO for
2
NO reduction with CO in the presence of O and SO . Conditions:
for 1 h for NO reduction with CO in the presence of O2 and
SO2. The catalytic activity clearly depends on the pretreat-
ment conditions. Higher activity was achieved for Ir/SiO2
reduced with H2–H2O/He gas, whereas Ir/SiO2 pretreated
with O2/He showed little activity for NO reduction. XRD
patterns of Ir/SiO2 treated with oxidizing or reducing gases
are given in Fig. 3. After the treatment with oxidizing gas
(Fig. 3(a)), distinct XRD peaks assignable to IrO2 were ob-
served, whereas diffraction lines due to Ir0 were observed
after the reduction (Fig. 3(b)). Accordingly, NO reduction
with CO proceeds mainly on Ir0 sites.
2
2
NO = 1000 ppm, CO = 6000 ppm, O = 5%, SO = 20 ppm, H O = 6%,
2
2
2
−3
W/F = 0.0267 g s cm . (Q) treated in flowing 5% O /He at 873 K,
2
(!) treated in flowing 10% H –6% H O/He at 873 K.
2
2
3.3. Catalytic performance of Ir/SiO2 for NO reduction
with CO
3.3.1. Effect of pretreatment condition
Fig. 2 shows the catalytic activity of Ir/SiO2 pretreated in
a flow of either 5% O2/He or 10% H2–6% H2O/He at 873 K

M. Haneda et al. / Journal of Catalysis 229 (2005) 197–205
201
Fig. 4. Effect of O concentration on (A) NO conversion to N + N O and (B) CO conversion to CO for NO reduction with CO in the presence of SO over
2
2
2
2
2
−3
Ir/SiO catalyst. Conditions: NO = 1000 ppm, CO = 6000 ppm, O = 0–10%, SO = 20 ppm, H O = 6%, W/F = 0.0267 g s cm . (Q) 0% O , (!) 1%
2
2
2
2
2
O , (2) 5% O , (E) 10% O .
2
2
2
3.3.2. Effect of O2 concentration
From a practical viewpoint, the effect of O2 concentration
is an interesting issue. Fig. 4 presents the effect of O2 con-
centration on the activity of Ir/SiO2 for NO reduction with
CO in the presence of SO2. When O2 was not present in
the reaction gas, NO reduction did not take place at tem-
peratures below 573 K, and the NO conversion gradually
increased with increasing reaction temperature above this
temperature. It is noteworthy that the NO conversion at tem-
peratures below 773 K was significantly increased by the
addition of 1% O2, indicating that O2 is necessary for NO
reduction with CO in the presence of SO2. However, a fur-
ther increase in O2 concentration to 10% caused a decrease
in the NO conversion. Since CO conversion also decreased
with increasing O2 concentration, the negative effect of too
much O2 is not due to an increase in undesirable CO com-
bustion. It is likely that most of the Ir0 species that act as
active catalyst components are oxidized to IrO2 during the
reaction.
Fig. 5. Effect of SO concentration on the activity of Ir/SiO for
2
2
NO reduction with CO in the presence of O . Conditions: NO =
2
1000 ppm, CO = 6000 ppm, O = 5%, SO = 0–20 ppm, H O = 6%,
2
2
2
−3
W/F = 0.0267 g s cm . (P) 0 ppm SO , (") 2 ppm SO , (1) 20 ppm
2
2
SO .
2
3.3.3. Effect of SO2 concentration
Fig. 5 shows the effect of SO2 concentration on the activ-
ity of Ir/SiO2 for NO reduction with CO in the presence of
O2. In the absence of SO2, selective reduction of NO with
CO did not occur. It should be noted that only a trace of SO2
(2 ppm) is sufficient to enhance deNOx activity. However, no
change in the NO conversion was observed when SO2 was
increased to 20 ppm.
Since the promotional effect of SO2 may be related to
the change in the supported iridium species, the response of
the NO conversion over Ir/SiO2 to an intermittent feed of
20 ppm SO2 was examined at 573 K. The results are given
in Fig. 6. It is clear that the conversion of NO was rapidly
increased after the introduction of SO2. The subsequent re-
moval of SO2 caused a gradual reduction in the conversion
of NO, and the initial low steady-state activity was resumed
in 4 h. This means that the effect of coexisting SO2 was al-
most completely lost after removal of SO2 from the reaction
Fig. 6. Response of NO conversion to intermittent feed of 20 ppm SO over
2
Ir/SiO at 573 K. Conditions: NO = 1000 ppm, CO = 6000 ppm, O = 5%,
2
2
−3
SO = 0 or 20 ppm, H O = 6%, W/F = 0.0267 g s cm . (!) NO conver-
2
2
sion to N , (Q) NO conversion to N O.
2
2

202
M. Haneda et al. / Journal of Catalysis 229 (2005) 197–205
gas, suggesting that the catalytically active sites for NO re-
duction are created by interaction with coexisting SO2 on the
catalyst surface.
reaction with or without SO2 and the FT-IR spectra of CO
species adsorbed to Ir/SiO2 in the presence of O2 and SO2.
3.4.1. Study of crystallite structure by XRD
Fig. 7 shows XRD patterns for Ir/SiO2 after use in the
reaction with or without SO2. It is apparent that only the
diffraction peaks assigned to IrO2 were observed in XRD
patterns for Ir/SiO2 samples after use in the reaction, mean-
ing that Ir metal particles are almost completely oxidized
during the reaction, irrespective of coexisting SO2. This re-
sult suggests that coexisting SO2 does not directly affect the
crystallite structure of iridium.
3.4. Influence of SO2 on the structure of iridium supported
on SiO2
The fact that NO conversion gradually decreased to the
initial steady state after the removal of SO2 from the reaction
gas, as illustrated in Fig. 6, suggests that SO2 interacts with
the catalyst, resulting in a change in the crystallite structure
and/or surface state of iridium. To gain information on this,
we measured the XRD patterns of Ir/SiO2 after use in the
3.4.2. Study of iridium surface by FT-IR following CO
adsorption
The oxidation state of the iridium surface was investi-
gated by FT-IR with CO as a probe molecule. This method is
useful for estimating the electron density of iridium. Fig. 8
shows FT-IR spectra for CO species adsorbed on Ir/SiO2,
recorded in different compositions of flowing gas at 523 K.
When 0.6% CO/He was exposed to Ir/SiO2 (Fig. 8A(a)),
a strong IR band at 2079 cm−1 assignable to CO linearly
bonded to Ir0 sites [15–17] was observed. The exposure of
CO in the presence of O2 to Ir/SiO2 gave rise to an IR band
at 2107 cm−1 (Fig. 8A(b)). A shift of the IR band to a higher
wavenumber is often observed when CO is adsorbed on an
oxidized metal surface [18–20]. Therefore, the IR band at
2107 cm−1 can be ascribed to linearly bonded CO adsorbed
on Irδ+ sites [20,21], indicating that the surface of iridium is
almost completely oxidized in the presence of O2.
On the other hand, when CO was exposed to Ir/SiO2 in
the presence of O2 and SO2 (Fig. 8A(c)), two distinct IR
bands assignable to Ir0–CO and Irδ+–CO were observed at
Fig. 7. XRD patterns of Ir/SiO (a) before and (b) after use in the reaction
2
0
without SO and (c) with SO . (Q) for IrO , (!) for Ir .
2
2
2
Fig. 8. (A) FT-IR spectra of CO adsorbed on Ir/SiO after the exposure of (a) 0.6% CO/He, (b) 0.6% CO–5% O /He and (c) 0.6% CO–5% O –20 ppm SO /He
2
2
2
2
at 523 K for 30 min. (B) FT-IR spectra of CO adsorbed on Ir/SiO after introducing 20 ppm SO into the 0.6% CO–5% O /He flowing for (a) 0, (b) 15 and
2
2
2
(c) 30 min.

M. Haneda et al. / Journal of Catalysis 229 (2005) 197–205
203
2074 and 2107 cm−1, respectively. This clearly indicates
that coexisting SO2 participates in the stabilization of Ir0
sites on which NO reduction with CO proceeds. It should
be noted that the introduction of SO2 into CO–O2/He flow-
ing gas caused the appearance of an IR band at 2074 cm−1
due to Ir0–CO, and its band intensity increased with time on
stream (Fig. 8B). Accordingly, the role of coexisting SO2 is
not only to stabilize but also to create Ir0 sites in an oxidizing
atmosphere. More detailed studies on how SO2 promotes the
reduction of iridium surface are now in progress that are us-
ing a surface science approach and a single-crystal model
catalyst. In brief, the disproportionate occurrence of SO2
troduced (spectrum (c)), the IR bands due to NO and CO at
1842 and 2080 cm−1 shifted to 1798 and 2060 cm−1, respec-
tively. The shift of the NO band to a lower wavenumber due
to surface interaction with CO was reported for Rh/Al2O3
[23] and Ir/Al2O3 [24]. This shift is explained by the per-
turbing effect of adsorbed CO, suggesting the formation of
cis-type coordinated species of NO and CO with one metal
NO
CO
atom (M
) [23,24]. Accordingly, the IR bands at 1798
and 2060 cm−1 observed here can be attributed to the for-
NO
mation of Ir
species.
CO
It has been reported that the M
NO
CO
species can be de-
takes place on the Ir(111) surface: 3SO2(a) → 2SO3(a) +S(a)
.
SO3 is thermally desorbed, and the atomic sulfur reacts with
surface oxygen on iridium: S(a) +2O(a) → 2SO2(g). The irid-
ium surface, thus, reverts to its initial metallic state. More
detailed results will be reported elsewhere [22].
composed to give CO2 and nitride (M–N), which can react
with NO and CO to produce N2O and NCO, respectively
[24]. In fact, as can be seen in Fig. 9 (spectrum (c)), a very
weak IR band due to the NCO species adsorbed on iridium
(Ir–NCO) [25] was detected at 2176 cm−1 for Ir/SiO2. It is
noteworthy that Ir/SiO2 showed high activity for NO reduc-
3.5. Observation of surface species formed in NO
reduction over Ir/SiO2
tion by CO in the absence of O2 and SO2 (Table 1). This
NO
suggests that the Ir
species is a possible intermediate
CO
To investigate the effect of SO2 on the behavior of
surface-adsorbed species formed during NO reduction with
CO, in situ IR spectra were measured at 523 K, at which tem-
perature the catalytic activity is low enough for surface inter-
mediates to be observed. Fig. 9 illustrates in situ IR spectra
of adsorbed species on Ir/SiO2 in contact with a series of
gas flows containing some of the selected gas components.
In flowing NO/He (spectrum (a)), an IR band assignable to
NO linearly adsorbed on iridium (Ir–NOδ+) [15] was de-
tected at 1842 cm−1. Exposure to CO also gave an IR band at
2080 cm−1 due to CO species adsorbed on iridium (Ir–CO)
[15–17] (spectrum (b)). When a mixture of NO+CO was in-
for NO reduction by CO. The fact that the bands at 1804
and 2042 cm−1 disappeared completely with the addition of
O2 to the NO + CO mixture (spectrum (d)) also supports
this idea, since NO reduction by CO did not take place over
Ir/SiO2 in the presence of O2 (Table 1).
Interestingly, the addition of SO2 to the reaction gas again
caused a shift of the 1842 cm−1 band to lower wavenum-
bers and the appearance of IR bands at 2038 and 2058 cm−1
(spectrum (e)). These band features are very similar to those
obtained in the flow of NO + CO (spectrum (c)). To be ex-
NO
CO
act, the Ir
surface species can be produced during NO
reduction by CO in the presence of O2 and SO2. However,
the band features in spectrum (e) are slightly different from
those in spectrum (c). The 2038 cm−1 band was observed
in the presence of O2 and SO2, suggesting that new adsorp-
tion sites for CO are created by the interaction of O2 and
SO2 over iridium, as described above (Section 3.4). Such in-
teraction may affect the electron donation from Ir to NO by
adsorption of SO2 and/or SO3, resulting in a shift of the N–O
vibrational frequency to a slightly higher value (1842 cm−1
)
than that detected for NO + CO (1798 cm−1; spectrum (c)).
From the above-mentioned results, we can conclude that
coexisting SO2 appears to play an important role in the for-
NO
mation of the Ir
surface species, resulting in significant
CO
activity enhancement (Table 1). This is probably due to the
fact that Ir0 sites can be created by coexisting SO2, as de-
scribed in the previous section. Specifically, the formation of
NO
the Ir
surface species takes place only on Ir0 sites, after
CO
which various reactions, leading to the formation of N2, con-
secutively proceed. On the other hand, CO oxidation is likely
Fig. 9. In situ IR spectra of adsorbed species formed on Ir/SiO in a
2
flow of (a) NO, (b) CO, (c) NO + CO, (d) NO + CO + O , and (e)
to occur predominantly on Irδ+ sites, because Irδ+ sites do
NO
2
NO + CO + O + SO at 523 K for 30 min. Conditions: NO = 1000 ppm,
2
2
CO = 6000 ppm, O = 5%, SO = 20 ppm.
not have the ability to form Ir
surface species.
2
2
CO

204
M. Haneda et al. / Journal of Catalysis 229 (2005) 197–205
that there is another route of formation of N2O. Concern-
ing N2O formation during the NO reduction, Burch et al.
[26,27] and Roberts [28] proposed a route via formation of
dimer (NO)2-type species as a favorable mechanism of N2O
formation over Pt catalysts:
2NO → (NO)2(a) → N2O + O(a)
.
(5)
If the formation of dimer (NO)2 species and subsequent
decomposition to N2O were very rapid, no evolution of
14N15NO would be observed. Accordingly, the formation of
N2O in the NO reduction with CO over Ir/SiO2 would fol-
low Eq. (5).
4. Conclusions
Ir/SiO2 shows very good activity with respect to NO re-
duction with CO in the presence of O2 and SO2, giving it
a potential for application to NOx removal from diesel and
lean-burn engine exhaust gas. Coexisting SO2 appears to
play an important role in creating and stabilizing the Ir0 sites
15
Fig. 10. Product responses when NO is replaced with an equal quantity of
14
NO in a NO–CO–O –SO /He mixture for Ir/SiO at 523 K. Conditions:
2
2
2
14 15
NO = 1000 ppm, CO = 6000 ppm, O = 5%, SO = 20 ppm.
N N ("),
2
2
15
14 15
15
N
(2),
N NO (P), N O (E).
2
2
on which NO reduction proceeds, resulting in the formation
NO
3.6. Proposed reaction pathway
of the Ir
species as a possible reaction intermediate.
CO
The formation of N2 and N2O from NO takes place via the
recombination of dissociated N atoms (N(a) + N(a) → N2)
and the formation of dimer (NO)2-type species (2NO →
(NO)2(a) → N2O + O(a)), respectively.
From the FT-IR measurements, cis-type coordinated
NO
CO
species of NO and CO with one iridium atom (Ir
) are
considered to be a possible intermediate for the selective re-
duction of NO with CO in the presence of O2 and SO2 over
Ir/SiO2. The participation of the same twin species of NO
and CO as reaction intermediates has been reported for the
NO + CO reaction over Rh/Al2O3 [23] and Ir/Al2O3 cat-
alysts [24]. According to these reports, NO reduction with
CO proceeds as follows:
Acknowledgment
The authors are grateful to Mr. Makoto Oishi for taking
the TEM microphotographs of the samples.
NO
CO
Ir
→ Ir–N + CO2,
(1)
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