The selective catalytic reduction of NO by CO (CO-SCR) over Ir-Ba/WO3-SiO2 was studied. Physical mixing of the It catalyst with SiO2, which is inert for CO-SCR, enhanced NO conversion, and NO conversion increased with increasing amounts of SiO2 in the physical mixture. HNCO was formed over the physical mixture at low oxygen concentrations. The dependence of CO-SCR activity on contact time in 0.5% O-2 suggested that HNCO was an intermediate species. In contrast, Ir catalysts that were not mixed with SiO2 exhibited only trace amounts of HNCO formation, even at low O-2 concentrations. Temperature-programmed desorption of HNCO from non-mixed and mixed Ir catalysts, which were pretreated in H-2 followed by H2O, revealed that the former had strong adsorption sites for NH3 formed by hydrolysis of HNCO, whereas the latter exhibited a smaller amount of these strong adsorption sites. The It catalyst showed high NH3 oxidation capacity under NH3-SCR conditions. The observed enhancement of CO-SCR activity over the mixed It catalyst was suspected to result from the decrease in the number of NH3 adsorption sites, which may have caused NH3 to be oxidized to NO. (C) 2010 Elsevier BM. All rights reserved.