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 O-2 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 O-2 is essential for NO reduction to occur. The role of coexisting SO2 was considered to be not only to stabilize but also to create Ir-0 sites in an oxidizing atmosphere. FT-IR measurements suggested that a cis-type coordinated species of NO and CO on one iridium atom (Irdrop(CO)(NO)) was an intermediate for NO reduction by CO. Although the Irdrop(CO)(NO) species completely disappeared with the addition of O-2 to the reaction gas, the presence of coexisting SO2 caused a reappearance of the Irdrop(CO)(NO) species. A reaction mechanism in which N-2 and N2O are produced via the recombination of dissociated N atoms (N(a) + N(a) --> N-2) and the formation of dimer (NO)(2)-type species (2NO --> (NO)(2(a)) --> N2O + O-(a)), respectively, is proposed. (C) 2004 Elsevier Inc. All rights reserved.