146
Chemistry Letters 2000
Catalytic Activity of Ir for NO-CO Reaction in the Presence of SO2 and Excess Oxygen
Masaru Ogura, Aya Kawamura, Masahiko Matsukata, and Eiichi Kikuchi*
Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
(Received November 8, 1999; CL-990950)
catalysts for NO reduction by C3H6 as a reductant.8,9 They
have claimed that the Ir catalyst supported on MFI zeolite con-
taining framework Co, Fe, and Al which are not ion-
exchanged, exhibited a significant stability against hydrother-
mal treatment at 700 °C. It is, however, generally known that
one of the reasons for deactivation of zeolite-based catalysts in
HC-SCR is due to dealumination from the zeolite framework.10
In this study, silicalite-1 having a MFI topology without Al in
the framework was used for the support of Ir, and the effect of
SO2 on the catalytic activities of Ir catalysts for NO-CO reac-
tion in the presence of excess oxygen was investigated.
Catalysts used in this study were Ir on three kinds of sup-
ports, SiO2 (amorphous, Fuji Silicia Corp.), Al2O3 (γ, Shokubai
Kasei Corp.), and silicalite (Tosoh Corp.). Ir was supported on
SiO2 and silicalite by adsorption in an aqueous solution of
[IrCl(NH3)5]Cl2. Firstly, the Ir compound was dissolved in
distilled water, and the pH was adjusted to 10 with a 10%
NH4OH solution. Then, the support was added to the solution,
which was stirred for 24 h at room temperature. After filtra-
tion, the Ir containing material was dried overnight in an air
oven at 110 °C. Ir/Al2O3 catalyst was prepared by impregnat-
ing the Al2O3 in a H2IrCl6 solution. The Ir content in every
catalyst was 0.02 wt%, as determined by XRF. Catalysts were
pre-treated prior to a reaction: temperature was raised at a rate
of 5 °C/min from room temperature to 500 °C in a 20% O2/He
stream, kept at 500 °C for 1 h, and cooled to 400 °C in a He
stream, followed by reduction at that temperature for 1 h in
flowing H2. Catalytic activity tests were performed by flowing
1000 ppm NO, 7500 ppm CO, 1-10% O2, and 0 or 150 ppm
SO2 in balancing He at a feed rate of 100 cm3/min onto 0.1 g
catalyst(GHSV: ca.40,000 h-1). Under the reaction conditions
employed in this study, N2O was hardly detected; therefore, the
catalytic activity was evaluated by the amount of NO convert-
ed into N2, determined by use of a chemiluminescence NOx
analyzer and a gas chromatograph.
Figure 1 shows the catalytic activities of various Ir cata-
lysts for NO reduction by CO in the presence of 1% O2 and in
the absence of SO2. Ir/silicalite gave slightly higher conver-
sions of NO in lower temperature range of 300-370 °C than
Ir/SiO2. It is noted that the selectivity of CO toward NO
reduction on Ir/silicalite, as expressed by the molar ratio of
reacted NO/reacted CO, is much higher than that on Ir/SiO2: at
365 °C, 0.30 and 0.13 for Ir/silicalite and Ir/SiO2, respectively.
Figure 2 illustrates the effect of SO2 on the NO conver-
sions over Ir/silicalite, Ir/SiO2, and Ir/Al2O3 at 400 °C. In the
initial run for 30 min without SO2, Ir/SiO2 showed higher NO
conversion than Ir/silicalite, and Ir/Al2O3 showed much lower
activity than Ir/silicalite. As shown in the following 30 min
run with 150 ppm SO2, the catalytic activity of Ir/silicalite was
hardly affected by SO2. Moreover, NO conversion on Ir/sili-
calite was stable over the period of 20 h in the presence of SO2.
On the other hand, NO conversion on Ir/SiO2 dropped by addi-
Catalytic performance of Ir catalysts for reduction of nitric
oxide with carbon monoxide in the presence of SO2 and excess
oxygen was investigated. NO was selectively reduced with CO
on Ir/silicalite in an oxidizing atmosphere containing 1% to
10% O2 in the temperature range of 300-500 °C. The catalytic
activity was scarcely influenced by 150 ppm SO2.
Catalytic reduction of nitric oxide (NO) in exhausts from
stationary combustors and vehicles is a desirable method to
apply in a practical usage. A three-way catalyst, composed of
Pt, Rh, and/or Pd as active centers, facilitates NO removal from
gasoline engine exhaust by use of hydrocarbons and carbon
monoxide as reducing agents. However, it can be used only in
a narrow window near the stoichiometry, where oxidizing mol-
ecules are balanced by reducing molecules. Because of the
lean-burn combustion system, NO reduction in the exhaust
from diesel engines must always be carried out in an oxidizing
atmosphere.1 Therefore, high selectivity of reductants toward
NO reduction is required for the catalytic process, and the
three-way catalyst is not feasible to apply for the diesel engine
exhaust. Actually, NO is emitted in the exhaust gas from vehi-
cles of diesel engine without any catalytic treatment.
Moreover, the gas contains particulate matters, which bring us
a problem of cancer.
Reports in the last decade have shown that many types of
catalysts, such as Cu-zeolites,2,3 or other transition metals on
zeolites and oxides supports4 can work in selective reduction of
NO by hydrocarbons (HC-SCR) in the presence of excess oxy-
gen. Zeolite-supported catalysts are attractive, because of their
wider range of temperature where they can reduce NO at high-
er conversion levels and more selectively than alumina and the
other supported catalysts. Most of the works using those types
of catalysts, however, have employed milder reaction condi-
tions than in the actual exhaust, such as lower space velocities,
higher concentrations of NO and hydrocarbons, lower concen-
trations of oxygen, and no H2O and SO2 in the reactant feed.
For practical clean-up of diesel exhausts, high selectivity
toward NO reduction even in the presence of excess oxygen is
indispensable, and much attention must be paid for the effect of
SO2 on the catalytic performance.5 One of the authors has pre-
viously reported that a very small amount of Ir(0.02 wt%) on
SiO2 can catalyze NO reduction with CO in the presence of
O2.6 Furthermore, it is noted that oxygen is needed to promote
the NO reduction, and that 3000 ppm of NO was selectively
reduced by 1% of CO in the oxidizing atmosphere up to 2% of
O2 coexisted. Utilization of CO as a reducing agent has advan-
tages for practical application to diesel engine exhaust, because
the engine works at relatively lower temperatures and the
exhaust from diesel engine contains comparable or higher con-
centration of CO than the gasoline engine exhaust.7 Recently
Nojima et al. reported the catalytic performance of supported Ir
Copyright © 2000 The Chemical Society of Japan