Characterization of Surface Oxygen Species on Zinc Oxide Revealed by the ESR Technique and the Transient Response Method*

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The nahure and reactivity of the oxygen species adsorbed on zinc oxide have been studied in detail over the temperature range room temperature-230℃ by using the ESR technique, the transient response method and an electrical conductivity measurement of the catalyst. The results obtained from the dynamic procedures have simultaneously suggested that there are three different kinds of oxygen species (O_2^-，O^- and O (non-ionic form)) on the surface during reaction. O_2^-is assigned by the ESR technique using the g-values g_<xx>=2.0499, g_<yy>=2.0030 and g_<zz>=2.0093, and it exists stably on the surface below 190℃. It disappears easily on exposure to CO, suggesting an induced charge transfer from O_2^- to the catalyst. The existence of O^- and O is predicted by the transient behaviour of gaseous O_2 and electrical conductivity, and the two species commonly react with CO apparently obeying first order kinetics. The mode of the transient response curves of the CO_2 formed shows a typical type VI curve based on the classification of the mode of response curves. From this result, it is presumed that the reaction consists of two different paths : one is the reaction of gaseous CO and O^- and the other is the reaction of CO and O. The apparent activation energy is 104 kJ/mol for the over-all reaction
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