NO reactions over Ir-based catalysts under oxygen-rich conditions

A series of experiments with various Ir-based catalysts for the NO reactions under oxygen-rich conditions have been conducted using a temperature programmed reaction (TPR) technique. The results indicate that the oxidation of NO to NO₂ and the reduction of NO to N₂ or N₂O occur over Ir-based catalysts under oxygen-rich conditions. The catalytic activity for the NO reactions increases with the increasing of Ir content. Furthermore, support materials also have important effects on the catalytic activity. When the Ir content is lower than 0.1 wt.%, the catalytic activity for the NO reactions is dependent on the characteristics of support materials and decreases in order of Ir/ZSM-5 > Ir/γ-Al₂O₃ > Ir/SiO₂. When the Ir content is higher than 0.1 wt.%, the catalytic activity for the NO oxidation is correlated with Ir dispersion on the surface of support materials and decreases in order of Ir/ZSM-5 > Ir/SiO₂ > Ir/γ-Al₂O₃, while the catalytic activity for the NO reduction is attributed to the adsorbed-decomposition of NO₂ and decreases in order of Ir/γ-Al₂O₃ > Ir/SiO₂ > Ir/ZSM-5. Compared with Pt catalyst, Ir catalyst has a similar activity for the NO oxidation but is more effective for the NO reduction.