Positive effect of coexisting SO2 on the activity of supported iridium catalysts for NO reduction in the presence of oxygen

Silica-supported iridium catalyst showed excellent activity for NO reduction with H2 in the presence of O2 and SO2. This reaction took place only on Ir and Rh/SiO2, whereas on Pt, Pd and Ru/SiO2 SO2 showed inhibiting effect on NO reduction. SiO2 was the only effective support for Ir. The most outstanding feature was that the coexistence of O2 and SO2 is essential for NO reduction to occur. The NO reduction activity did not change much in the presence of 10–150 ppm of SO2, and H2O did not affect the activity, either. Concerning O2 concentration, only a small amount of O2 was sufficient for the activity increase and further increase of O2 concentration decreased NO conversion. The increase of Ir loading and H2 amount enhanced NO conversion. Accordingly, NO conversion of more than 40% was obtained in the presence of 10% O2 at a space velocity of 50,000 h−1. The FT-IR study revealed that the presence of O2 and SO2 promoted the adsorption of NO and formation of NHx species on Ir/SiO2.