Enhancement of activity of Ir catalysts for the selective catalytic reduction of NO by CO

Article abstract of DOI:10.1246/cl.2006.450

The activity of Ir catalysts for selective catalytic reduction of nitrogen monoxide by carbon monoxide was drastically enhanced by combining iridium and tungsten oxide on a silica support. Copyright

Full text of DOI:10.1246/cl.2006.450

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Chemistry Letters Vol.35, No.4 (2006)
Enhancement of Activity of Ir Catalysts for the Selective Catalytic Reduction of NO by CO
ꢀ
Tetsuya Nanba, Satoru Shinohara, Junko Uchisawa, Shouichi Masukawa, Akihiko Ohi, and Akira Obuchi
Institute for Environmental Management Technology,
National Institute of Advanced Industrial Science and Technology (AIST),
16-1 Onogawa, Tsukuba 305-8569
(Received January 16, 2006; CL-060063; E-mail: tty-namba@asit.go.jp)
The activity of Ir catalysts for selective catalytic reduction
The mixture was dried at 383 K and calcined at 773 K for 4 h.
The supports thus prepared are designated WO3–SiO2(a), where
(a) denotes the weight percent of WO3 in the support. Typically
0.5 wt % iridium was loaded on each support by an impregnation
method using Ir(NH3)6(OH)3, Ir(NH3)6(NO3)3, H2IrCl6, or
Ir(NO3)4 as a precursor. The impregnated samples were dried
and then calcined at 773 K for 4 h in air. Details of the reactor
system and analytical equipment used have been described else-
of nitrogen monoxide by carbon monoxide was drastically en-
hanced by combining iridium and tungsten oxide on a silica sup-
port.
Selective catalytic reduction (SCR) of NO by CO is a prom-
ising method for controlling NOx (NO þ NO2) emissions from
diesel engines, where catalysts are required to work in a net
oxidizing atmosphere. Ogura et al. reported that Ir/silicalite at
very low Ir loading exhibits high activity, with and without
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where. The powdered catalyst (0.1 g) was diluted with granular
SiC (0.25–0.6 mm), and a total volume of 0.4 mL of the mixture
was packed into a quartz tube reactor (inner diameter, 8 mm).
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SO2. Haneda et al. reported that Ir/SiO2 shows high activity
The flow rate of the reactant gas was 225 mL min . The reac-
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only in the presence of SO2. Shimokawabe et al. suggested that
Ir/WO3 and Ir/ZnO show high activities but at a relatively low
tant gas consisted of 500 ppm NO, 5000 ppm CO, 10% O2,
and 1% H2O, with He or N2 as a balance gas. The catalyst
was pretreated in a 10% H2/He flow at 873 K for 1 h. The activ-
ity was measured at temperatures ranging from 673 to 473 K in
steps of 10 K. The catalysts’ BET surface area (Nikkiso, Model
4232), surface morphology, and elemental analysis (Hitachi,
FE-SEM; S-4700) were measured.
Figure 1 shows the temperature dependence of the CO-SCR
activity of Ir/WO3–SiO2(10), Ir/SiO2, and Ir/WO3. The maxi-
mum NOx conversions for Ir/SiO2 and Ir/WO3 were 11 and
39%, respectively. In contrast, Ir/WO3–SiO2 exhibited 86%
maximum NOx conversion and 89% maximum N2 selectivity
(N2=ðN2 þ N2OÞ ꢂ 100) at 533 K. These results clearly indicate
that the combination of WO3 with SiO2 drastically enhanced the
CO-SCR activity. The decrease in N2 selectivity above 550 K
was probably due to rapid consumption of the reductant CO
3
space velocity and a low oxygen concentration (2%). Higher-
performing catalysts should be sought for practical applications.
In consideration of regulations requiring future reductions in
sulfur concentrations in diesel fuel, we combined WO3 with
SiO2 as a support material, and the resulting catalyst exhibited
high performance as a CO-SCR catalyst in model diesel ex-
hausts, in the absence of SO2.
The peroxopolytungstic acid solution was prepared by add-
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ing 15% hydrogen peroxide to metallic tungsten powder. NH3
was then added to the solution to adjust the pH to 8.5. A prede-
termined amount of colloidal silica sol (Catalysis & Chemical
Inc. Co., Ltd.; Cataloid S-20L, containing 20 wt % SiO2) was
mixed with the tungsten solution at various WO3/SiO2 ratios.
1
00
W/Ir molar ratio
0
20 40 60 80 100 120 140 160
1
00
8
6
4
2
0
0
0
0
0
80
60
40
20
0
4
50
500
550
600
650
700
Temperature /K
0
20
40
60
80
100
Figure 1. Temperature dependence of CO-SCR activity. Ir pre-
cursor, Ir(NH3)6(OH)3; weight of catalysts, 0.1 g; feed gas: 500
ppm NO, 5000 ppm CO, 10% O2, 1% H2O, balance He; flow
rate, 225 mL min . Symbols indicate NOx conversion for 0.5
wt % Ir/WO3–SiO2(10) ( ), 0.5 wt % Ir/WO3 ( ), and 0.5
wt % Ir/SiO2 ( ), and N2 selectivity of 0.5 wt % Ir/WO3–
SiO2(10) ( ).
WO loading /wt %
3
Figure 2. Dependence of CO-SCR activity on WO3 loading in
the support. Ir precursor, Ir(NH3)6(OH)3; weight of catalysts,
0.1 g; feed gas: 500 ppm NO, 5000 ppm CO, 10% O2, 1%
H2O, balance He; flow rate, 225 mL min . Symbols indicate
NOx conversion ( ) and N2 selectivity ( ).
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Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.4 (2006)
451
Table 1. Effect of Ir precursor on CO-SCR activity
Table 2. BET surface area
WO3 content in support/wt %
Precursor
Ir(NO3)4
NOx conv./%
N2 selec./%
Area unit
44
43
64
72
84
88
86
89
0
10
30
50
80
100
H2IrCl6
2
m /g
2
165
165
153
170
107
153
70
140
16
80
6
—
Ir(NH3)6(NO3)3
Ir(NH3)6(OH)3
m /g-SiO2
Catalyst: 0.5 wt % Ir/WO3–SiO2(10). Reaction conditions as
in Figures 1 and 2, except that N2 was used as the balance gas.
and 553 K. Note that NOx conversions exceeded 80% when
the WO3 content in the support ranged from 1 to 30%, which
corresponds to a W/Ir molar ratio range of 1.7–50. Above
5
0% WO3 content, the NOx conversion was similar to that of
(a)
Ir/WO3. N2 selectivity exhibited the same trend as NOx conver-
sion. This result suggests that a relatively low WO3 content was
favorable for enhanced CO-SCR activity.
The choice of the iridium precursor also influenced the CO-
SCR activity. Table 1 shows that the maximum NOx conversion
and N2 selectivity over 0.5 wt % Ir/WO3–SiO2(10) prepared
with various iridium precursor. Ir/WO3–SiO2(10) prepared with
Ir(NH3)6(OH)3 showed the highest activity.
Figure 3 shows a SEM image of Ir/WO3–SiO2(10) prepared
with Ir(NH3)6(OH)3 as a precursor. WO3 particles are abundant
on the surface in the region near the center of the photograph
(
Figure 4a, line). EDX analysis of this region (Figure 4b) clearly
shows that Ir is present on WO3 rather than on SiO2. This result
suggests that Ir/WO3–SiO2(10) consists mainly of Ir/WO3 dis-
persed on SiO2. Table 2 lists the BET surface areas of WO3–
SiO2 at various composition ratios. Although the surface area
per gram of support decreases linearly with increasing WO3 con-
tent, the surface area per gram of SiO2 decreases only slightly at
WO3 contents lower than 50%. This result suggests that at WO3
levels up to 50%, WO3 was finely dispersed over the SiO2
surface without reduction in the high surface area of the SiO2.
Because Ir/WO3 showed high CO-SCR activities, we con-
cluded that the increase in the WO3 surface area due to disper-
sion of small particles on SiO2 enhanced the catalytic activity.
Further investigation of the role of Ir and WO3 in CO-SCR is
in progress.
1
0µm
(b)
Si
W
Ir
References
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Distance / µm
Figure 3. (a) SEM image of 0.5 wt % Ir/WO3–SiO2(10) and
b) EDX analysis of the area indicated by the line in (a).
3
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(
1
76.
and would increase the rate of partial reduction of NO to N2O.
Figure 2 shows the effect of WO3 content in the support on
the maximum NOx conversion and N2 selectivity. The maximum
NOx conversion and N2 selectivity were attained between 523
5
6
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Published on the web (Advance View) March 25, 2006; DOI 10.1246/cl.2006.450