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Chemistry Letters Vol.33, No.5 (2004)
Selective Catalytic Reduction of NO by CO over Supported Iridium and Rhodium Catalysts
Masahide Shimokawabe and Noriyoshi Umeda
Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628
(Received December 15, 2003; CL-031238)
The catalytic reduction of NO with CO was investigated in
was mounted on loosely packed quartz wool. The concentrations
of N2, N2O, O2, CO, and CO2 in the outflow gas were deter-
mined using gas chromatographs (Shimadzu 6A and Ohkura
701) with porapak Q and molecular sieve 5A columns. The con-
centration of NO2 was monitored using a UV–vis spectropho-
tometer (Hitachi Model 100-10).
the presence of oxygen over 16 supported metallic catalysts. Ir/
WO3, Ir/ZnO, and Rh/Al2O3 showed the pronounced activity
even in excess amount of oxygen. The effect of preparation con-
ditions on the activity of Ir catalysts was further studied and it
was found that Ir/WO3 prepared by loading of H2IrCl6 at
0
.5 wt % Ir is the most active for the NO reduction with CO.
Table 1 summarizes the conversion of NO to N2 and/or N2O
in the catalytic reduction of NO (1000 ppm) by CO (1%) in the
presence of O2 (2%) over various metal catalysts supported on
Al2O3 obtained at 473, 523, and 573 K. The pronounced activi-
ties for the reduction of NO are observed over Rh/Al2O3 even at
lower temperature (473 K), while the selectivity for N2O forma-
tion is higher than that for N2 formation. The NO conversion for
Rh/Al2O3 decreases with an increase in the reduction tempera-
ture. On the other hand, at higher temperature (573 K), Ir/Al2O3
and Cu/Al2O3 reveal high activities for N2 formation in CO re-
duction of NO. On the basis of the results shown in Table 1, the
effect of support on the catalytic activity of Ir and Rh catalysts is
further investigated.
Three way catalysts are designed to reduce emissions of car-
bon monoxide, nitrogen oxides, and uncombusted hydrocarbons.
Under lean conditions, certain hydrocarbons have been proved to
act as selective reductants, while CO and H2 have not been re-
garded as selective reductant under lean conditions, since they
are oxidized by O2 rather than by NO under an oxygen-rich at-
mosphere. Recently, Burch et al. have found that the reduction of
NO by H2 could carried out in an oxidizing conditions over Pt
catalysts under an oxygen-rich atmosphere. Ogura et al. found
that Ir/silicate could catalyze NO reduction by CO even in the
presence of excess oxygen. Wang et al. reported that among
Pt, Pd, Rh, and Ir noble metal catalysts, only Ir/ZSM-5 catalyst
exhibited high activity for NO reduction by CO in the presence
of excess oxygen.8,9 These are very interesting information since
automotive exhausts always contain certain amount of CO.
However, the catalysts for NO reduction with CO in the presence
of excess oxygen have been received only a little attention and
many questions still exist.
In the present study, the reduction of NO with CO by 7 met-
als, Ag, Cu, Fe, Ir, Pd, Pt, and Rh, supported on metal oxides has
been investigated in the presence of excess O2. The effect of 6
metal oxide supports on the catalytic activity of Ir and Rh were
also investigated. The influence of the loading amount of metal
and the starting Ir compound on the activity of the highly active
Ir catalysts was further studied.
1
–4
5
,6
7
Table 1. The conversion of NO to N2 or N2O in the reduction of
NO by CO over various metals supported on Al2O3
473 K
523 K
573 K
Catalyst
N2/% N2O/% N2/% N2O/% N2/% N2O/%
Ag/Al2O3
Cu/Al2O3
Fe/Al O3
0
0
16.8
0
0
0
1.0
11.5
0.6
0
47.7
0
3.4
0
7.8
0.8
5.1
5.0
4.4
12.3
41.5
13.3
0
2
Ir/Al O3
2
0
0
14.6
4.6
0
Pd/Al O3
2
3.7
0
0
0
0
Pt/Al O3
2
0
53.0
11.1
26.5
1.6
11.1
0
19.9
Rh/Al O3 21.9
2
Reactants: NO (1000 ppm) – CO (1%) – O2 (2%)
Table 2 summarizes the conversion of NO to N2 and/or N2O
obtained over Ir and Rh supported on Al2O3, Ga2O3, In2O3,
WO3, ZnO, and ZSM-5. At 473 K, Rh/ZnO also shows the pro-
nounced activities for NO reduction together with Rh/Al2O3. At
573 K, Ir catalysts supported on WO3, ZnO, In2O3, and Ga2O3
reveal excellent activities for CO reduction of NO and these Ir
catalysts show a higher selectivity for N2 formation. Especially,
Ir/WO3 shows the highest value of N2 selectivity at 523 K. On
the basis of the results shown in Table 2, the effect of loading
amount of Ir is further investigated over Ir/WO3.
Table 3 summarizes the conversion of NO to N2 and/or N2O
together with that of CO obtained at 523 K over Ir/WO3 with
0.1, 0.3, 0.5, and 1.0 wt % Ir loaded. The conversion of NO to
N2 increases with an increase in the loading amounts of Ir and
the N2 formation increases drastically between 0.3 and
0.5 wt % Ir. The maximum value (82.1%) of N2 formation is ob-
tained at 0.5 wt % Ir. On the other hand, CO conversion is less
than 24.5% at Ir loading below 0.3 wt % , while CO conversion
exhibits 100% above 0.5 wt % Ir. From these results, it is found
Rh or Ir supported on Ga2O3, In2O3, WO3, ZnO, and ZSM-
, Ir supported on Ga2O3, and Ag, Cu, Fe, Pd, Pt, Rh, or Ir sup-
5
ported on Al2O3 were prepared by an impregnation method.
Al2O3, Ga2O3, In2O3, WO3, ZnO, or Na-ZSM-5 were impreg-
nated with aqueous solution of acetate of Cu, chloride of Ag,
Pd or Ir, nitrate of Fe or Rh, or ammonium nitrate of Pt in a rotary
evaporator at 343 K. These catalysts were further calcined in air
for 2 h at 773 K. Na-ZSM-5 (SiO2/Al2O3 = 23.8) was supplied
from Tosoh Co.Ltd. Metal loading amounts were 0.1, 0.3, 0.5,
and 1.0 wt %. Unless otherwise stated, all the experiments were
carried out using 0.5 wt % metal loading catalyst.
The reactant gases used were NO (1000 ppm), CO (1%) and
O2 (2%), and they were diluted by He. Prior to the runs, the cat-
alysts were treated in the reactor at 773 K for 2 h and cooled to
the reaction temperature in a stream of He. The reaction was car-
ꢀ
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ried out in a conventional flow reactor at W/F of 0.6 g s cm
and at 473, 523, and 573 K. The reactor was made of 9-mm di-
ameter Pyrex glass tubing in which the catalyst sample of 0.5 g
Copyright Ó 2004 The Chemical Society of Japan
Shimokawabe, Masahide
