A highly active Ir/WO3 catalyst for the selective reduction of NO by CO in the presence of O2 or O2 + SO2

Article abstract of DOI:10.1246/cl.2005.1426

Ir/WO₃ catalyst is highly active for the reduction of NO by CO even in the presence of either O₂ or O₂ + SO₂. However, the activity of Ir/WO₃ is fairly lowered by the presence of SO₂ alone. It is believed that the active sites lose their activity by the adsorption of SO₂ but O₂ promotes the desorption of SO₂ from these sites as suggested by TPD, thus the negative effect of SO₂ being suppressed by the coexistence of O₂. Copyright

Full text of DOI:10.1246/cl.2005.1426

1426
Chemistry Letters Vol.34, No.10 (2005)
A Highly Active Ir/WO3 Catalyst for the Selective Reduction of NO
by CO in the Presence of O2 or O2 þ SO2
Masahide Shimokawabe,ꢀ Mihiro Niitsu, Hironori Inomata, Nobuhiro Iwasa, and Masahiko Arai
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628
(Received July 19, 2005; CL-050935)
Ir/WO3 catalyst is highly active for the reduction of NO by
3 h and cooled to the reaction temperature. The reactant gases
used were NO (1000 ppm), CO (1%) and O2 (0–8%) and they
were diluted with He. In order to investigate an effect of SO2,
100 ppm of SO2 was added to the reactant gas in the transient
mode. Unless otherwise stated, all the experiments were carried
out using 2% O2. The concentrations of N2, N2O, O2, CO, and
CO2 in the outflow gas were determined using gas chromato-
graphs (Shimadzu 8A and 6A) with porapak Q and molecular
sieve 5A columns. The concentration of NO2 was monitored us-
ing a UV–vis spectrophotometer (Hitachi Model U-1100). TPD
experiments of SO2 adsorbed on Ir/WO3 and WO3 were carried
out to investigate the desorption behavior of SO2, using a BEL
Japan, Inc., TPD-1-AT detected by Q-MASS detector.
The reactions were run at temperatures of 473–673 K and
the reactions were slow at 523 K or below under the conditions
used. Table 1 summarizes the values of conversion of NO to N2
or N2O, and CO over Ir/WO3 measured at higher temperatures.
The pronounced activity more than 60% NO conversion is ob-
served above 573 K except in the presence of SO2. The highest
value of NO conversion is observed in the reaction of NO–CO
without both O2 and SO2. CO conversion obtained in the pres-
ence of O2 or O2 þ SO2 reaches to 100% since the oxidation
of CO proceeds very rapidly in the presence of excess O2. It
may be a reason for that the conversion of NO (to N2 and
N2O) is saturated around 60% above 623 K in the presence of ex-
cess O2. It is found that SO2 remarkably inhibits the NO–CO re-
action, while this reaction is not inhibited by the presence of O2
or SO2 þ O2. The values of total NO conversion for NO–CO–O2
reaction in the presence and absence of SO2 obtained at 573–
673 K are almost identical, while the selectivity values for N2
and N2O are different. The selectivity for N2O increases but that
for N2 decreases to almost the same extent by introducing SO2.
Namely, the coexisting O2 was able to mask the negative effect
of SO2. Possible mechanisms for the formation of N2 and N2O
from NO is considered to be the recombination of N atoms ad-
sorbed on adjacent Ir sites (2N(a) ! N2) and the reaction of an
adsorbed N atom and an adsorbed (or gaseous) NO molecule,
respectively. Introducing SO2, which is adsorbed on Ir sites as
proved by TPD (Figure 2), would reduce the couples of the ad-
jacent N atoms, thus decreasing the conversion to N2 but increas-
CO even in the presence of either O2 or O2 þ SO2. However, the
activity of Ir/WO3 is fairly lowered by the presence of SO2
alone. It is believed that the active sites lose their activity by
the adsorption of SO2 but O2 promotes the desorption of SO2
from these sites as suggested by TPD, thus the negative effect
of SO2 being suppressed by the coexistence of O2.
The selective catalytic reduction of NO in oxygen-rich at-
mosphere has recently attracted extensive attention for removing
NOx emitted from diesel and lean-burn engines. Under lean con-
ditions, certain hydrocarbons have been proved to act as selec-
tive reductants,1–3 while CO and H2 have not been regarded as
selective reductant since they are oxidized by O2 rather than
by NO. Furthermore, the catalysts are often deteriorated by the
presence of SO2 in diesel exhaust. Recently, it has been reported
that Ir supported on silicate4 and ZSM-55,6 can catalyze NO re-
duction by CO even in the presence of excess oxygen. Hamada et
al.7–9 have reported that Ir/SiO2 showed no NO reduction activ-
ity in the absence of SO2, while the presence of SO2 drastically
promotes NO reduction. This is quite a favorable characteristic
for the treatment of diesel exhaust. Previously, the authors have
studied the catalytic reduction of NO with CO in the presence of
excess O2 over various supported metal catalysts.10 The pro-
nounced activity was obtained with Ir/WO3, Ir/ZnO, and Rh/
Al2O3 catalysts, among which the first one is the most active.
In the present study, the influence of SO2 on the reduction of
NO has further been investigated for the Ir/WO3 catalyst.
Ir/WO3 was prepared by an impregnation method. WO3
.
support was prepared by a decomposition of (NH4)10W12O41
5H2O in air at 773 K for 3 h. WO3 was impregnated with aque-
ous solution of H2IrCl6 in a rotary evaporator at 343 K. The
catalyst was further calcined in air at 773 K for 2 h. Ir loading
was 5.0 wt %.
The reaction was carried out in a conventional flow reactor
at W/F of 0.06 g s cmꢁ3 and at 423–673 K. The reactor was made
of 6 mm diameter Pyrex glass tubing in which the catalyst sam-
ple of 0.05 g was mounted on loosely packed quartz wool. Prior
to the runs, the catalyst was treated in a stream of He at 773 K for
Table 1. The conversiona to N2 and N2O and of CO in the reaction of NO and CO in the presence and absence of O2 and/or SO2 with a
5 wt % Ir/WO3
Temperatureb
/K
NO + CO
N2O CO
NO + CO + O2
NO + CO + SO2
NO + CO + O2 + SO2
N2
N2
N2O
CO
N2
N2O
CO
N2
N2O
CO
573
623
673
74
80
87
7
4
0
8
12
11
66
53
55
15
8
4
100
100
100
7
10
8
6
7
9
1
3
4
41
49
50
42
22
9
100
100
100
aIn %. Conversion to N2 (or N2O) = (N2 (or N2O) at outlet)/(NO at inlet); conversion of CO = (CO2 at outlet)/(CO at inlet) in the
absence or presence of O2. bReaction temperature.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.10 (2005)
1427
100
(1) WO3
,
,
: without SO2
: with SO2
80
60
40
20
0
(2) Ir/WO3
N2
(3) Ir/WO3
800
N2O
300
400
500
600
700
900
Temperature / K
0
2
4
6
8
Figure 2. TPD curves of SO2 adsorbed on WO3 and Ir/WO3
collected during heating in pure He (1 and 2) and in O2 (2%)
and He (3).
O2 concentration / %
Figure 1. The relationship between O2 concentration and the
conversions of NO and CO in the reaction between NO and
CO over Ir/WO3 at 573 K in the presence and absence of SO2.
by 2% O2 þ He for 15 min, followed by TPD in the same O2 þ
He stream (curve 3). A broad desorption peak is observed around
350 K for the support alone (curve 1). For curve (2), two peaks
are seen around 350 and 650 K, which may correspond to de-
sorption of SO2 adsorbed on WO3 and Ir species, respectively.
When TPD was conducted in the presence of O2, a different re-
sult was obtained (curve 3); the peak assigned to the desorption
from Ir shifts to lower temperature by about 60 K. Namely O2
can promote the desorption of SO2 from surface Ir sites and this
is a reason for O2 to remove the negative effect of SO2 deactivat-
ing those sites.
The influence of O2 and SO2 on the catalytic reduction of
NO with CO has been investigated for an Ir/WO3 catalyst,
which is highly active for this reduction in the absence of those
foreign gases. It is shown that NO can be reduced by CO even in
the presence of either O2 or O2 þ SO2. It is also seen that the cat-
alyst shows high activity even in excess O2 atmosphere such as 5
to 8% O2 in the presence and absence of SO2. It could be consid-
ered that the resistance of Ir/WO3 to oxidation is very useful for
sulfur free lean-burn engines in addition to diesel exhaust.
ing the conversion to N2O.
Figure 1 shows the relationship between the O2 concentra-
tion and the conversion of NO to N2 and N2O in the presence
and absence of SO2. The conversion to N2 obtained in the
NO–CO reaction without O2 decrease drastically by introducing
SO2. When the reaction is carried out in the presence of O2 be-
tween 0.3 and 1%, the conversion to N2 increases up to 80%
while the conversion to N2O shows less than 15%. When SO2
is introduced in the NO–CO reaction in the presence of O2 be-
tween 0.3 and 1%, the conversion to N2 decreases and that to
N2O increases inversely. It is found that the maximum conver-
sion of NO in the absence of SO2 is obtained at an O2 concen-
tration around 0.5%. When NO and CO are completely convert-
ed to N2 and CO2, respectively, according to an equation,
NO þ 10CO þ 9/2O2 ! 1/2N2 þ 10CO2, 0.45% O2 is the sto-
ichiometric concentration under the present conditions (0.1%
NO and 1% CO), which is close to the O2 concentration for
the maximum NO conversion observed. When more than
0.45% O2 is added, the oxidation of CO may be promoted but
this may decrease the NO reduction by CO. Certainly, when
the reaction is carried out in the presence of O2 above 1%, CO
conversion steadily shows 100% and conversion to N2 decreases
with increase in O2 concentration, while conversion to N2O in-
creases inversely. The conversion to N2O is further increased by
introducing SO2. As well known in the literature,1–4 6–16% O2
and 1–500 ppm SO2 are usually contained in diesel exhaust. It
is seen that Ir/WO3 catalyst shows still high activity even in ex-
cess O2 atmosphere, for example, 70% NO conversion for 5%
O2. It could be considered that the resistance of Ir/WO3 to oxi-
dation is very useful for SCR in oxygen-rich atmosphere.
Figure 2 illustrates the TPD curves of SO2 adsorbed on WO3
and Ir/WO3. For SO2 adsorption, the samples were exposed to a
stream of 1% SO2 diluted with He at 50 cm3 minꢁ1 for 60 min at
room temperature. They were flushed by flowing He for 60 min
and heated in pure He (50 cm3 minꢁ1) at a rate of 10 K minꢁ1
(curves 1 and 2) or by flowing pure He for 45 min and further
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Published on the web (Advance View) September 17, 2005; DOI 10.1246/cl.2005.1426