Reaction of NO over iridium catalysts with Excess oxygen

A series of iridium-supported catalysts with various Ir loadings were prepared by impregnation method. The support materials used were ZSM-5, γ-Al₂O₃, and SiO₂. The Ir loadings examined were 0.02%, 0.1%, 1%, and 5% (w). The behavior of the catalysts for the NO reaction was analyzed by means of temperature programmed reaction (TPR) experiments. The reactions of NO over iridium include the oxidation of NO to NO₂ and the reduction of NO to N₂ or N₂O. The iridium catalysts promote the reactions of NO and the catalytic activity increases with the increase of the Ir loadings. Further, support materials have a little effect on the catalytic activity. When the loading was less than 0.1%, the catalytic activity was found to be dependent on the property of support materials and the activity order was Ir/ZSM-5>Ir/gamma;-Al₂O₃>Ir/SiO₂; when the loading was higher than 0.1%, the order of catalytic activity for NO oxidation was Ir/ZSM-5>Ir/SiO₂>Ir/γ-Al₂O₃, which was correlated with Ir dispersion on the surface of support materials, and the order of catalytic activity for NO reduction was Ir/γ-Al₂O₃>Ir/SiO₂>Ir/ZSM-5, which was attributed to the adsorbed- dissociation of NO₂. Ir/γ-Al₂O₃ catalyst was more beneficial for the NO reduction than Pt/γ-Al₂O₃ catalyst.