Selective catalytic reduction of nitrogen monoxide with H2 or CO as reductant in presence of SO2

Ir/SiO2 and Rh/SiO2 catalysts showed excellent activity for NO reduction with CO or H-2 in the presence Of 02 and SO2. Ir/SiO2 catalyzed NO reduction with both CO and H-2, whereas Rh/SiO2 catalyzed reduction only with H2. The most important characteristic was that coexistence of O-2 and SO2 is essential for NO reduction to occur. Surface science investigation using Ir(111) single crystal model catalyst revealed that the atomic sulfur was formed via disproportionation of SO2 and reacted with oxygen adsorbed on the surface to form SO2, which desorbed from the surface. Therefore, the iridium surface reverted to its initial metallic state. FT-IR measurements on Ir/SiO2 also indicated that the SO2 both stabilized and created Ir-0 sites in the oxidizing atmosphere. The addition of Li and Ba to Ir/SiO2 and Zn to Rh/SiO2 was quite effective to enhance the catalytic activity for NO reduction in the presence of O-2 and SO2 by acting as oxidation retardant to prevent deactivation of the active Ir and Rh metals supported on SiO2.