Unique properties of Ir/ZSM-5 catalyst for NO reduction with CO in the presence of excess oxygen

The reduction of NO with CO in the presence of excess oxygen was investigated over different noble metal catalysts for probing the relationship between catalytic properties and adsorption behaviors. Among the four precious metal catalysts investigated, Ir/ZSM-5 was found to be the only active one for NO reduction with CO under lean conditions. With the decreasing of the Ir content, higher NO conversion and CO selectivity was obtained. Temperature-programmed reaction (TPR) studies of NO/H₂/O₂ and NO/CO/O₂ showed that the Pt/ZSM-5 was active when H₂ was used as the reductant, whereas, the Ir/ZSM-5 was active when CO was the reducing agent. This difference is due to the different mechanisms of the two reactions. Temperature-programmed desorption (TPD) of NO, CO and O₂ showed that NO could dissociate more easily over the Ir/ZSM-5 than on the Pt/ZSM-5, while the oxidation of CO by O₂ proceeded more rapidly on the Pt/ZSM-5 than on the Ir/ZSM-5. The presence of excess O₂ inhibited drastically the dissociation of NO, which is considered as the key step for the NO–CO reaction. The high dissociation rate of NO over the Ir/ZSM-5 is visualized as the key factor for its superior high activity in NO reduction with CO under lean conditions.