Kinetics and mechanism of NO reduction with CO on Ir surfaces

The adsorption and thermal reactivity of NO and CO and the kinetics of the NO reduction with CO on Ir surfaces were studied using X-ray photoelectron spectroscopy, polarization modulation infrared reflection–absorption spectroscopy, and temperature programmed desorption. The NO adsorption and dissociation activity was strongly dependent on the Ir surface structure. The NO dissociation activity of the Ir planes decreased in the order (100) > (211) ≫ (111). In contrast, the type of the CO adsorption site was independent of the Ir surface structure. The activity of Ir(111) for N2 and CO2 production from the NO + CO reaction was low compared with the activities of Ir(100) and Ir(211). The kinetic data for an Ir/SiO2 powder catalyst were similar to data obtained for Ir(211). The order of the turnover frequencies for N2 and CO2 formation for the Ir planes was in good agreement with the order for NO dissociation activity, and this agreement indicates that the catalytic activity for NO reduction was dependent on NO dissociation. A kinetic study of the elementary steps indicated that the rate-limiting step for NO reduction with CO was the NO dissociation step.