Promoting effect of coexisting H2O on the activity of Ir/WO 3/SiO2 Catalyst for the selective reduction of NO with CO

Article abstract of DOI:10.1246/cl.2008.830

The catalytic activity of Ir/WO₃/SiO₂ for the selective reduction of NO with CO (CO-SCR) in the presence of excess O ₂ was significantly increased by the presence of coexisting H ₂O, whereas NO reduction hardly

Full text of DOI:10.1246/cl.2008.830

830
Chemistry Letters Vol.37, No.8 (2008)
Promoting Effect of Coexisting H2O on the Activity of Ir/WO3/SiO2 Catalyst
for the Selective Reduction of NO with CO
Masaaki Hanedaꢀ and Hideaki Hamada
Research 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
(Received April 14, 2008; CL-080388; E-mail: m.haneda@aist.go.jp)
The catalytic activity of Ir/WO3/SiO2 for the selective re-
duction of NO with CO (CO-SCR) in the presence of excess
O2 was significantly increased by the presence of coexisting
H2O, whereas NO reduction hardly occurred in the absence of
H2O. Coexisting H2O plays an important role preventing cata-
lyst deactivation caused by oxidation of the catalytically active
Ir sites at high temperatures.
steady-state activity 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-programmed reduction (TPR) measurements were
performed to investigate the reducibility of the catalysts.
Figure 1 shows the activity of Ir/WO3/SiO2 for CO-SCR
when H2O concentration was varied from 0 to 10%. When
H2O was not present in the reaction gas, NO reduction hardly
occurred in the entire temperature range, and CO conversion
to CO2 was low. On the other hand, the presence of 0.75%
H2O caused a significant increase in NO conversion as well as
CO conversion. It was also found that the catalytic activity of
Ir/WO3/SiO2 increased with increasing H2O concentration up
to 6%. This indicates that coexisting H2O plays an important role
for the catalytic activity.
The selective reduction of NO in oxidizing atmosphere
currently attracts great interest both in applied and fundamental
research. Recently, we reported that CO acts as an effective
reductant for NO reduction over Ir/SiO2 catalyst in the presence
of O2 and SO2.1 The most outstanding feature of this catalytic
reaction is that the coexistence of O2 and SO2 is essential for
NO reduction to occur. By combining surface science techniques
using single-crystal model catalyst and real catalyst,2 it was
found that the coexisting SO2 stabilizes the catalytically active
reduced Ir site.
In addition to SO2, H2O is another important coexisting
constituent in diesel exhaust. The influence of H2O on the
selective reduction of NO with hydrocarbons (HC-SCR) has
been extensively studied, and activity enhancement by H2O
has been reported.3–5 In this case, the role of H2O is considered
to remove carbonaceous materials deposited on the catalyst
surface and/or to improve the reaction selectivity by suppression
of undesirable hydrocarbon oxidation. Therefore, it is of interest
to study how coexisting H2O influences the catalytic activity
for the CO-SCR reaction from not only fundamental but also
practical points of view. In the present work, we have investigat-
ed in detail the effect of coexisting H2O on the catalytic activity
of Ir/WO3/SiO2, which, as we already found, shows high
activity for CO-SCR.6
The response of NO conversion to the intermittent feed of
H2O was examined at 300 ꢁC. As shown in Figure 2, when
H2O was removed from the reaction gas, NO conversion slightly
decreased with time but did not reach the value given in Figure 1
(4%). The subsequent introduction of H2O did not recover the
NO conversion to the initial level. This result suggests that
100
80
60
40
20
0
200
300
400
500
600
Temperature/°C
Ir/WO3/SiO2 was prepared by consecutive impregnation
100
80
60
40
20
0
.
method. (NH4)10W12O41 5H2O and citric acid were dissolved
in distilled water, to which commercial SiO2 (Fuji Silysia Chem-
icals, Cariact G-10) was added. The mixture was dried in air at
110 ꢁC overnight and calcined in flowing air at 500 ꢁC for 5 h.
The resulting WO3/SiO2 powder was then impregnated with a
solution of Ir(NO3)3 (Ishifuku Metal Industry Co., Ltd.), fol-
lowed by drying at 110 ꢁC overnight and calcination at 600 ꢁC
for 6 h in air. The loading of Ir and WO3 was fixed at 0.5 and
10 wt %, respectively. The catalytic activity test was carried
out using a flow reactor system by passing a reaction gas mixture
containing NO (500 ppm), CO (3000 ppm), O2 (5%), SO2
(1 ppm), and H2O (0–10%) diluted in He at a rate of 90
cm3 minꢂ1 over 0.04 g of catalyst, which had been pretreated
in situ in a flow of He at 600 ꢁC for 2 h. The effluent gas was
analyzed by gas chromatography. The reaction temperature
was changed from 600 to 200 ꢁC in steps of 20–50 ꢁC, and the
200
300
400
500
600
Temperature/°C
Figure 1. Effect of H2O concentration on the activity of
Ir/WO3/SiO2 for NO reduction with CO. Conditions: 500 ppm
NO, 5% O2, 3000 ppm CO, 1 ppm SO2, 0–10% H2O, W=F ¼
0:027 g s cmꢂ3. ( ) 0% H2O, ( ) 0.75% H2O, ( ) 1.5% H2O,
(
) 6% H2O, ( ) 10% H2O.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.8 (2008)
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100
80
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231
338
413
f
60
350
420
e
d
40
229
232
329
389
with H2O
20
with H2O
no H2O
444
379
c
0
0
100
200
300
Time on stream/min
b
236
Figure 2. Response of NO conversion to N2 + N2O over
Ir/WO3/SiO2 to the intermittent feed of H2O at 300 ꢁC. Condi-
tions: 500 ppm NO, 5% O2, 3000 ppm CO, 1 ppm SO2, 0 or 6%
a
0
100
200
300
400
500
600
H2O, W=F ¼ 0:027 g s cmꢂ3
.
Temperature/°C
Figure 4. TPR profiles of (a) Ir/SiO2, (b) WO3/SiO2, (c) Ir/
WO3/SiO2, (d) Ir/WO3/SiO2 after He treatment, (e) Ir/WO3/
SiO2 after the reaction in the absence of H2O, (f) in its presence.
100
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60
40
20
0
observed for Ir/WO3/SiO2 treated with He (profile-d). This
suggests the slight reduction of Ir sites and the creation of strong
interaction between Ir and WO3 by He treatment. As given in
profile-e, an increase in the area of the low-temperature peak
and a shift of the middle-temperature peaks to high temperature
were observed for Ir/WO3/SiO2 after use in the reaction in
the absence of H2O. This TPR profile is very similar to that
for Ir/WO3/SiO2 oxidized at 600 ꢁC (profile-c). The surface of
Ir/WO3/SiO2 seems to be oxidized under the reaction condi-
tions in the absence of H2O. On the other hand, Ir/WO3/SiO2
after use in the presence of H2O (profile-f) gave very similar
TPR profile with that for He treated sample. Namely, the TPR
profile with a small low-temperature peak and middle-tempera-
ture peaks shifted to low-temperature was obtained. These
results lead us to the consideration that coexisting H2O plays a
role to keep a portion of the Ir sites in the reduced state and in
the strong Ir–W interaction during the reaction.
Since the reaction gas employed in this study contains CO
and H2O, the formation of H2 by water gas shift (WGS) reaction
can be expected. In fact, a certain amount of CO2 was formed
when a gas mixture of CO and H2O was exposed to Ir/WO3/
SiO2 catalyst at temperatures above 400 ꢁC, suggesting the
possibility of WGS reaction. We also observed that the activity
of Ir/WO3/SiO2 for NO reduction with CO in the absence of
H2O is increased by addition of H2. In conclusion, coexisting
H2O plays an essential role for NO reduction to occur over
Ir/WO3/SiO2 catalyst through stabilization of the catalytically
active Ir sites in reduced state by H2 produced on-site via
WGS reaction.
200
300
400
500
600
Temperature/°C
Figure 3. Comparison of NO conversion measured in the
presence of H2O from 600 ꢁC ( ) and in the absence of H2O
from 400 ꢁC ( ). Conditions are the same as for Figure 2.
H2O does not directly participate in the reaction as a reactant. In
order to get information on the role of H2O, the NO reduction
activity in the absence of H2O was measured from 400 ꢁC after
pretreatment in He at 600 ꢁC. As shown in Figure 3, in this case,
the NO conversions were very similar to those in the presence of
H2O. We recently reported that the active sites of Ir/WO3/SiO2
for NO reduction with CO are Ir–WOx (x ¼ 2:92{3) species in
which Ir metal interacts strongly with tungsten oxide.6 This
means that the presence of reduced Ir sites is necessary for the
NO reduction activity. On the other hand, reduced Ir is oxidized
in the presence of O2 at temperatures above 500 ꢁC.7 Therefore,
coexisting H2O can be considered to prevent catalyst deactiva-
tion caused by oxidation of the catalytically active Ir sites at
the temperatures above 400 ꢁC.
In order to confirm this idea, the oxidation state of Ir in
Ir/WO3/SiO2 was examined by TPR, the results of which are
shown in Figure 4. Ir/SiO2 and WO3/SiO2 as the reference
samples, which were oxidized in flowing 20% O2/N2 at
600 ꢁC before measurement, gave a peak at 236 ꢁC and above
450 ꢁC, respectively. These peaks are due to the reduction of
IrO2 and WO3. In the TPR profile of Ir/WO3/SiO2 oxidized at
600 ꢁC (profile-c), broad H2 consumption peaks were detected
at 379 and 444 ꢁC in addition to these two peaks. These peaks
would be ascribed to the reduction of oxygen present at the
Ir–WO3 interface, because no such peaks were observed for
Ir/SiO2 and WO3/SiO2. It is apparent that a significant decrease
in the area of the low-temperature peak and a shift of the middle-
temperature peaks to lower temperature (329 and 386 ꢁC) were
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Published on the web (Advance View) July 5, 2008; doi:10.1246/cl.2008.830