CO is one of the most practical reductants for the removal of
NO because it is present in exhaust emissions from vehicles or is
relatively easily produced by engine control. However, CO was not
regarded as an effective reductant for SCR until recently because
CO is easily consumed by the reaction with O2 when supported
noble metal catalysts are employed. CO-SCR was first reported by
Tauster and Murrell [48]. They measured the catalytic activity of
0.1% Ir/Al2O3(0.1 g) for CO-SCR using a reaction gas mixture composed of 0.2% NO, 1.0% CO and 0.75% O2diluted in He at a flow rate of
100 L h
−1
. They reported that the NO conversion was as high as 90%
at 400
◦
C. This indicates that NO reacts preferentially with CO rather
than with O2. The interesting catalytic performance of Ir/Al2O3was
explained by a higher probability of adsorbing NO molecules onto
a surface site compared with O2molecules. They also emphasized
that 0.001% Ir/Al2O3is also highly active for the CO-SCR, although
the effective temperature window shifts to a region about 100
◦
C
higher.
After the first report by Tauster and Murrell, various materials
have been investigated in order to develop effective catalysts for
CO-SCR. The CO-SCR performance of various catalysts reported so
far is summarized in Table 5. In 2000, Ogura et al. [49] reported that
NO can be successfully reduced to N2with CO over 0.02% Ir/silicalite
catalyst under the reaction conditions of 1000 ppm NO, 7500 ppm
CO and 1% O2. In addition to Ir/silicalite, Ir/SiO2and Ir/Al2O3can
also effectively catalyze the CO-SCR reaction, although the latter
is not so active. The above-mentioned reports suggest that Ir is a
promising catalytic component for CO-SCR.