Promotional effect of SO2 on the activity of Ir/SiO2 for NO reduction with CO under oxygen-rich conditions

The effect of coexisting SO2 on the activity of silica-supported noble metal catalysts for the selective reduction of NO with CO in the presence of O2 was investigated. Pt/SiO2, Rh/SiO2, and Pd/SiO2 showed little catalytic activity for NO reduction, irrespective of coexisting SO2. Although Ir/SiO2 showed no NO reduction activity in the absence of SO2, the presence of SO2 drastically promoted NO reduction. A comparison of the catalytic performance of Ir/SiO2 and Ir/Al2O3 in the presence of SO2 showed that Ir supported on SiO2 is more active than Ir on Al2O3. SiO2 was found to be a more effective support than Al2O3. The most outstanding feature of the reaction on the Ir/SiO2 catalyst was that the coexistence of SO2 and O2 is essential for NO reduction to occur. The role of coexisting SO2 was considered to be not only to stabilize but also to create Ir0 sites in an oxidizing atmosphere. FT-IR measurements suggested that a cis-type coordinated species of NO and CO on one iridium atom () was an intermediate for NO reduction by CO. Although the species completely disappeared with the addition of O2 to the reaction gas, the presence of coexisting SO2 caused a reappearance of the species. A reaction mechanism in which N2 and N2O are produced via the recombination of dissociated N atoms (N(a) + N(a) → N2) and the formation of dimer (NO)2-type species (2NO → (NO)2(a) → N2O + O(a)), respectively, is proposed.