Drastic enhancement of SCR of NO over Ir catalyst through formation of metallic iridium on Na-zeolite

Article abstract of DOI:10.1246/cl.2004.800

The co-existence of both CO and H₂ and the use of Na-zeolite supports leads to effective NO reduction below 500 K for the SCR of NO over low loaded (0.5 wt %) Ir catalyst, which was attributed to the formation of metallic iridium species.

Full text of DOI:10.1246/cl.2004.800

800
Chemistry Letters Vol.33, No.7 (2004)
Drastic Enhancement of SCR of NO over Ir Catalyst through Formation
of Metallic Iridium on Na-zeolite
Junji Shibata, Hisao Yoshida,y Atsushi Satsuma,ꢀ and Tadashi Hattori
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603
yResearch Center for Advanced Waste and Emission Management, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603
(Received April 12, 2004; CL-040395)
The co-existence of both CO and H2 and the use of Na-zeo-
lite supports leads to effective NO reduction below 500 K for the
SCR of NO over low loaded (0.5 wt %) Ir catalyst, which was at-
tributed to the formation of metallic iridium species.
Al2O3. The catalytic test was performed with a fixed-bed flow
reactor by passing a mixture of 1000-ppm NO, 3000-ppm CO,
5000-ppm H2, and 2% O2 in He at a rate of 100 cm3 minꢁ1 over
0.05-g catalyst. Prior to the experiment the catalyst was heated in
6.7% O2/He at 773 K for 1 h. After reaching a steady-state, the
effluent gas was analysed by a gas chromatograph and a chemi-
luminescence NOx analyser (Best BCL-100uH). Dispersion of Ir
catalysts before and after the SCR was measured with CO–H2 ti-
tration method.6 Ir LIII-edge XANES spectra of the samples
treated with exposing reaction gas mixtures were recorded in flu-
orescence mode by using Lytle detector at the BL-10B station7 at
KEK-PF with a Si(311) channel cut monochromator.
SCR of NO by CO and H2 was examined over Ir/H-MFI. As
shown in Table 1, NO conversions were very low for the SCR by
sole reductant. Maximum NO conversions at 673 K were 3% for
CO-SCR and 10% for H2-SCR, respectively. On the other hand,
the co-existence of CO and H2 increased NO conversion remark-
ably. Maximum NO conversion was 78% at 523 K, and NO was
also reduced effectively even below 500 K. Under the co-pres-
ence of CO and H2, NO conversion was remarkably higher than
the sum of those by individual reducants. In separate experi-
ments, NO conversion in CO (2000 ppm)-H2 (3000 ppm)-SCR
over Ir/H-MFI was 7% at 498 K, while those in H2 (5000
ppm)-SCR and CO (5000 ppm)-SCR were 4% and 0%, respec-
tively. These results indicate that there is a synergistic effect
of co-existence of CO and H2.
A selective catalytic reduction of NO in the presence of ex-
cess oxygen is a potential method to remove NOx from lean-burn
and diesel exhausts. Among a number of catalysts, Ir catalysts
are effective in the reduction of NO by not only unsaturated hy-
drocarbon,1–3 but also CO3,4 and hydrogen3 under lean-burn ex-
haust conditions. Moreover, Ir catalysts have been reported to
have moderate tolerance to water and SO2,1,4 and exhibit high
NO reduction activity rather in the presence of SO2.3 However,
Ir catalysts show lower NO reduction activity at lower tempera-
tures below ca. 500 K. Hence, the extension of active window of
Ir catalysts to lower temperatures is strongly desired. Recently,
Haneda et al. have reported that Ir/SiO2 exhibits about 10% of
NO conversion at 473 K for the SCR by CO in the presence of
SO2.5 However, loaded amount of Ir (5 wt %) was high. In this
study, we discover that Ir catalysts with low loaded amount
(0.5 wt %) show high activity below 500 K by co-existence of
CO and H2 and use of Na-zeolite supports. Moreover, by means
of Ir LIII-edge XANES and CO–H2 titration, the reason for high
activity is clarified to be low oxidation state and high dispersion
of iridium species.
Ir dispersion on Ir/H-MFI estimated from CO–H2 titration
was 7, 13, and 12% after SCR by CO (3000 ppm), H2 (5000
ppm) and CO (3000 ppm) + H2 (5000 ppm), respectively. This
result indicates that the co-presence of two reducing agents does
not cause a change in Ir dispersion. Figure 1A shows Ir LIII-edge
XANES of Ir/H-MFI after the SCR by CO and H2. XANES
spectrum after the SCR by only CO (spectrum b) and only H2
(spectrum c) had relatively large white line at 11220 eV and
was similar to that of IrO2, indicating Ir oxide was main species
Ir catalysts were prepared by impregnating supports with an
aqueous solution of IrCl4 followed by evaporation to dryness at
303 K and by calcination in air at 773 K for 4 h. Loaded Ir
amount was 0.5 wt %. Na-MOR, SiO2-Al2O3, SiO2, and MgO
were supplied by the Committee of Reference Catalysts, Catal-
ysis Society of Japan (JRC-Z-M15, JRC-SAL-2, JRC-SIO-8
and JRC-MGO-4 100A). H-MOR (Si/Al = 7.9), H-MFI (Si/
Al = 20) and Na-MFI (Si/Al = 13) were supplied by Tosoh Cor-
poration. Boehmite was employed as starting materials of Ir/
Table 1. NO conversion and N2 selectivity over Ir catalysts for the SCR by CO and H2
[CO]
/ ppm
[H2]
/ ppm
Ir dispersion
/ %a
NO conversion (N2 selectivity) / %
573 K
Catalysts
Ir/H-MFI
448 K
473 K
498 K
523 K
623 K
673 K
3000
0
0
5000
5000
5000
5000
5000
5000
5000
5000
5000
7
13
12
30
26
8.9
9.1
17
20
25
0 (—b)
3 (100)
14 (81)
25 (75)
38 (62)
11 (69)
5 (67)
0 (—)
3 (100)
40 (79)
50 (74)
64 (64)
63 (45)
13 (50)
8 (83)
0 (—)
4 (85)
2 (100)
8 (89)
3 (100)
6 (100)
52 (89)
12 (75)
29 (76)
62 (83)
17 (79)
24 (80)
26 (100)
1 (100)
3 (100)
10 (100)
30 (86)
7 (68)
3000
3000
3000
3000
3000
3000
3000
3000
78 (83)
41 (73)
57 (67)
75 (70)
36 (59)
22 (65)
23 (74)
0 (—)
71 (88)
22 (72)
44 (76)
70 (78)
35 (77)
38 (81)
38 (85)
0 (—)
Ir/Na-MFI
Ir/Na-MOR
Ir/SiO2
Ir/SiO2-Al2O3
Ir/H-MOR
Ir/Al2O3
Ir/MgO
6 (82)
12 (61)
9 (61)
45 (87)
12 (82)
12 (79)
17 (100)
1 (100)
2 (100)
0 (—)
0 (—)
2 (100)
0 (—)
a
b
measured with CO–H2 titration6 after SCR reaction. neither N2 nor N2O was detected.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.7 (2004)
801
on H-MFI. On the other hand, XANES spectrum after the SCR
under the co-existence of CO and H2 had small white line at
11218 eV and was similar to Ir metal (spectrum d). Therefore,
the co-presence of CO and H2 leads to reduction of Ir oxide to
metallic iridium.
tion for the oxidation state of iridium. Therefore, electronic state
of Ir catalysts after the SCR was estimated by integrated inten-
sity of white line at Ir LIII-egde XANES spectra, obtained by
curve-fitting analysis with an arctangent and a Gaussian func-
tion.8 Figure 2 shows the relationship between integrated inten-
sity of the white line and TOF of NO (NO reaction rate per sur-
face iridium atom). TOF of NO increased roughly with a
decrease in the white line intensity, that is, extent of oxidation
of Ir species. This relationship strongly supports that metallic iri-
dium is highly active species for NO reduction. Nakatsuji1 and
Nojima et al.2 have reported the high NO reduction activity of
metallic iridium for the SCR by unsaturated hydrocarbons.
Our present result reveals quantitatively the high NO reduction
activity of metallic iridium for the SCR by CO and H2. Higher
NO conversions on Ir/Na-zeolites than Ir/SiO2 and Ir/H-MFI
can be ascribed mainly to higher dispersion of iridium.
In order to design more active Ir catalyst, the SCR under the
co-existence of CO and H2 was carried out with various sup-
ports. Table 1 also shows NO conversion for the SCR by co-ex-
istence of CO and H2 at various temperatures. For all the cata-
lysts except for Ir/MgO, NO conversion increased with
reaction temperature, reached the maximum at 498–573 K, and
then decreased with further increase of temperature. Clearly,
NO conversion differed by support materials. Among Ir/zeo-
lites, Ir/Na-zeolites exhibited higher NO reduction activity than
Ir/H-MFI below 500 K. Particularly, NO conversion on Ir/Na-
MOR was 38% at 473 K. This low temperature catalytic activity
is much higher than that of 5 wt % Ir/SiO2 for the CO-SCR in the
presence of SO2 (NO conversion: 9.6%).5 On the other hand, Ir/
H-MOR showed NO reduction around 573 K, and little activity
below 500 K. Among Ir/metal oxides, Ir/SiO2 showed compara-
ble activity to Ir/H-MFI. Ir/SiO2-Al2O3 and Ir/Al2O3 had NO
reduction activity around 550 K. NO reduction activity of Ir/
MgO was very low over the whole reaction temperatures. The
order of NO reduction activity at 473 K was Ir/Na-MOR > Ir/
Na-MFI > Ir/H-MFI = Ir/SiO2 > Ir/SiO2-Al2O3 > Ir/H-
MOR > Ir/Al2O3 > Ir/MgO. N2 selectivity (N2 yield/(N2 yield
+ N2O yield)) on all catalysts was above 60% in the active tem-
perature range.
Ir metal
IrO2
100
80
60
40
20
0
h
f
g
6
e
c
d
a
b
8
10
12
(A)
(B)
a
a
White line intensity / a.u.
Figure 2. NO TOF for SCR under co-existence of CO and H2 over Ir
catalysts at 473 K, as a function of white line intensity estimated from
Ir LIII-edge XANES. Catalysts: (a) Ir/MgO, (b) Ir/Al2O3, (c) Ir/SiO2-
Al2O3, (d) Ir/H-MOR, (e) Ir/H-MFI, (f) Ir/SiO2, (g) Ir/Na-MFI and
(h) Ir/Na-MOR.
d
b
e
c
f
d
g
h
In summary, Ir/Na-zeolites, particularly Ir/Na-MOR, ex-
hibits excellent activity at 500 K for the SCR under the co-exis-
tence of CO and H2. This effect is attributed to the remarkable
formation of highly active metallic iridium species and high dis-
persion of iridium species.
i
i
11200
11220
11240 11200
11220
11240
X-ray energy / eV
This work was partly supported by a Grant-in-Aid from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. The X-ray absorption experiment was performed under
the approval of the Photon Factory Program Advisory Commit-
tee (Proposal No. 2001G093).
Figure 1. Ir LIII-edge XANES spectra of (a) IrO2, (b–d) Ir/H-MFI,
(e) Ir/Al2O3, (f) Ir/H-MOR, (g) Ir/Na-MOR, (h) Ir/SiO2, and (i) Ir
metal. The spectra b and c were measured after SCR reaction by only
CO and only H2, respectively. The spectra d–h were measured after
SCR reaction under co-existence of CO and H2.
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Figure 1B shows Ir LIII-edge XANES of various Ir catalysts
after the SCR under the co-existence of CO and H2. For Ir/
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spectra of Ir/Na-MOR (spectrum g) and Ir/SiO2 (spectrum h) as
well as Ir/H-MFI, showed white line at 11218 eV, which is sim-
ilar to a spectrum of Ir metal.
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Published on the web (Advance View) May 31, 2004; DOI 10.1246/cl.2004.800