Excellent promoting effect of Ba addition on the catalytic activity of Ir/WO3-SiO2 for the selective reduction of NO with CO

Article abstract of DOI:10.1246/cl.2006.420

The catalytic performance of Ba-doped Ir/WO₃-SiO₂ for the selective reduction of NO with CO in an oxidizing atmosphere was investigated. The NO reduction activity and durability of the Ir/WO ₃-SiO₂ catalyst were

Full text of DOI:10.1246/cl.2006.420

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Chemistry Letters Vol.35, No.4 (2006)
Excellent Promoting Effect of Ba Addition on the Catalytic Activity
of Ir/WO –SiO for the Selective Reduction of NO with CO
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Ã
Atsushi Takahashi, Tadahiro Fujitani, Isao Nakamura, Yasuhiro Katsuta, Masaaki Haneda, and Hideaki Hamada
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 305-8565
(Received January 27, 2006; CL-060118; E-mail: at-takahashi@aist.go.jp)
The catalytic performance of Ba-doped Ir/WO3–SiO2 for
to investigate the durability of catalysts, the activities were
measured by repeating the heating sequence from 140 to
600 C and the cooling sequence from 600 to 140 C. We also
examined the oxidation properties of the catalysts by means of
temperature-programmed oxidation (TPO).
the selective reduction of NO with CO in an oxidizing atmo-
sphere was investigated. The NO reduction activity and durabil-
ity of the Ir/WO3–SiO2 catalyst were drastically improved by
the addition of Ba. Ba played an important role to stabilize the
catalytically active Ir–WOx (x < 3) sites of Ir/WO3–SiO2 dur-
ing the reaction.
ꢀ
ꢀ
Figure 1 shows the NO conversion to N2 + N2O and CO
conversion to CO2 over Ba–Ir/WO3–SiO2 with different Ba
loadings, where the conversions indicate the initial catalytic
activities. In the absence of Ba, the NO reduction occurred at
ꢀ
The selective reduction of NO in an oxidizing atmosphere is
one of the most desirable technologies for removing NOx emit-
ted from diesel engines and lean-burn engines. Currently, the se-
lective reduction of NO using CO as a reductant attracts atten-
tion for which Ir catalysts are known to exhibit a high activity.
Ogura et al. reported that Ir/silicate gave a high NO conversion
temperatures below 450 C and a maximum NO conversion of
ꢀ
53% was obtained at 280 C. The NO reduction activity of
Ir/WO3–SiO2 was drastically promoted by the addition of Ba.
The maximum NO conversion over Ba–Ir/WO3–SiO2 (Ba/Ir
= 1/5) was about 2 times higher (94%) than that of Ir/WO3–
SiO2. Since the activity for the CO conversion was also increas-
1
ꢀ
in the presence of 1% O2. Haneda et al. reported that Ir/SiO2
shows a high activity under oxidizing atmosphere when SO2 co-
ed by the addition of Ba in the range of 240–300 C, It seems that
the enhancement of the NO reduction activity is due to the CO
activation as reductant by the addition of Ba.
2
exists. Simokawabe et al. also showed that the selective reduc-
3
tion proceeds effectively on Ir/WO3 and Ir/ZnO. Recently,
Nanba et al. found that the NO reduction activity of Ir/SiO2
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was drastically enhanced by the addition of W. However, the
development of catalysts to show higher activity and durability
in a wide temperature range is desired from a practical point
of view. Accordingly, we have investigated the activity of vari-
ous Ir/WO3-based catalysts for the selective reduction of NO
with CO in an oxidizing atmosphere and found that only Ba-dop-
ed Ir/WO3–SiO2 catalysts show a significantly high activity and
durability under wide reaction conditions. In this paper, we re-
port the excellent performance of Ba-doped Ir/WO3–SiO2 cata-
lysts with discussions of the roles of Ba additive in the activity.
.
(
NH4)10W12O41 5H2O and citric acid were dissolved in dis-
tilled water to which SiO2 (Fuji Silysia Chemicals, Cariact G-10,
2
À1
3
00 m Ág ) was added. The weight ratio of WO3:SiO2 was 1:9.
ꢀ
The mixture was dried in air at 110 C overnight and calcined in
ꢀ
air at 500 C for 4 h to obtain WO3–SiO2 support. Ir/WO3–SiO2
was prepared by impregnation of the WO3–SiO2 support with an
aqueous solution of H2IrCl6. The impregnated catalyst precursor
ꢀ
ꢀ
was dried at 110 C overnight, followed by calcination at 600 C
for 6 h in air. The loading of Ir metal was fixed at 5 wt %. Then,
Ba was doped to Ir/WO3–SiO2 using Ba(NO3)2 solution, fol-
ꢀ
ꢀ
lowed by drying at 110 C overnight, and calcined at 600 C
for 6 h in air. The loading of Ba was varied from 1/20 to 1/1
(
a fixed bed flow reactor. Prior to each reaction, the catalyst was
molar ratio of Ba/Ir). The catalytic activity was measured using
ꢀ
pretreated in a flow of 10% H2/He at 600 C for 2 h. The reaction
gas mixture consisting of 500 ppm NO, 3000 ppm CO, 5% O2,
Figure 1. Catalytic activities of Ba–Ir/WO3–SiO2. Reaction
conditions: 500 ppm NO, 5% O2, 3000 ppm CO, 6% H2O,
1 ppm SO , Ba/Ir ratio: 0 ( ), 1/20 ( ), 1/10 ( ), 1/5 ( ),
6
% H2O, 1 ppm SO2 and He balance was passed through the cat-
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À1
À1
alyst at a gas flow rate of 90 cm Ámin (SV = ca. 75,000 h ).
2
The effluent gas was analyzed by gas chromatography. In order
1/3 ( ), and 1/1 ( ).
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.4 (2006)
421
Figure 3. TPO profiles of Ir/WO3–SiO2 and Ba–Ir/WO3–SiO2
ꢀ
(
conditions: O2 (5%)/He, 10 C/min. Sample weight, 0.1 g.
Ba/Ir = 1/10). Pretreatment: 600 C, H2 (10%)/He, Operating
ꢀ
SiO2 was higher than that of Ir/WO3–SiO2, considering that
the oxidation of Ir metal during the reaction was suppressed
by Ba.
To get information on the oxidation property of Ir/WO3–
SiO2 and Ba–Ir/WO3–SiO2, we carried out TPO measurements
for the reduced catalysts (Figure 3). The O2 consumption peaks
ꢀ
appeared at 300 and 570 C for Ir/WO3–SiO2 were due to the
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oxidation of W and Ir, respectively.
The temperatures for
ꢀ
the oxygen consumption of both W and Ir shifted 50 C higher
by the addition of Ba, clearly indicating that Ir and W in Ba–
Ir/WO3–SiO2 are difficult to be oxidized than those in Ir/
WO3–SiO2.
Figure 2. Catalytic activities of Ir/WO3–SiO2 (
,
) in heating and cooling
) and Ba–
Ir/WO3–SiO2 (Ba/Ir = 1/10) (
,
sequences. Reaction conditions: 500 ppm NO, 5% O2, 3000
ppm CO, 6% H2O, 1 ppm SO2.
The promotional effect of W on the catalytic activity of Pt/
SiO2 for NO + CO reaction was reported by Regalbuto et al.9
They suggested that the Pt–WOx adliniation sites, which are
located on Pt particles decorated by partially reduced tungsten
The activities, which indicate the steady-state conversions,
on Ir/WO3–SiO2 and Ba–Ir/WO3–SiO2 (Ba/Ir = 1/10) during
the heating and cooling sequence were shown in Figure 2. On Ir/
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WO3–SiO2, the maximum NO conversion reached 65% at
oxides, have a high NO dissociation activity. Nanba et al. sus-
ꢀ
2
60 C in the heating sequence. However, the maximum NO
pected that it is necessary to oxidize the reduced Ir/WO3–SiO2
catalyst in order to obtain a high NO reduction activity, consid-
ering that Ir–WOx-like (x < 3) sites are formed by the reduc-
tion–oxidation treatment, which act as active site for NO reduc-
tion.
conversion was decreased to 30% in the cooling sequence, which
was 1/2 of that in the heating sequence. In addition, the temper-
ature giving the maximum NO conversion shifted from 260 to
ꢀ
3
50 C. After the heating and cooling sequences were repeated
several times, the maximum NO conversion significantly de-
creased to 10% (not shown). For the CO conversion, the active
temperature in the cooling sequence was 50–150 C higher than
The result of TPO measurement showed that Ba suppressed
the oxidation of not only Ir but also W, suggesting that the
partially reduced WOx was prevent from deep-oxidation in the
presence of Ba during the reaction. We thought that Ba plays
an important role in stabilizing the active sites, Ir–WOx species,
during the reaction. Now, we are investigating the role of Ba and
the active sites of the Ir/WO3-based catalysts in further detail.
ꢀ
that in the heating sequence. After repeating both sequences sev-
eral times, the active temperature range became higher, indicat-
ing that the CO oxidation activity decreased monotonously dur-
ing the reaction. CO oxidation reaction is well known to proceed
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mainly on metal surface, indicating that the decrease in the CO
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ꢀ
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ꢀ
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Published on the web (Advance View) March 18, 2006; DOI 10.1246/cl.2006.420