Enhancement of activity of Ir catalysts for selective catalytic reduction of NO with CO by physical mixing with SiO2

The selective catalytic reduction of NOx by CO (CO-SCR) over Ir–Ba/WO3–SiO2 was studied. Physical mixing of the Ir catalyst with SiO2, which is inert for CO-SCR, enhanced NOx conversion, and NOx 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% O2 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 O2 concentrations. Temperature-programmed desorption of HNCO from non-mixed and mixed Ir catalysts, which were pretreated in H2 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 Ir catalyst showed high NH3 oxidation capacity under NH3-SCR conditions. The observed enhancement of CO-SCR activity over the mixed Ir 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.