Abstract
By using thermal desorption (TD) gas analysis, oxygen-isotope exchange is found between 18O 2 gas and UV-irradiated MgO powders baked in high vacuum. At a peak temperature ( T p) 125 K, only 18O 2 TD takes place. At 200 K, TD's of 18O 2 and 16O 18O with a small amount of 18O 2 are observed. In the temperature range 550–800 K, a group of three TD peaks composed of 18O 2, 16O 18O and 16O 2 is obtained. The exchange ratios, R( 16O 18O) and R( 16O 2) defined respectively as the ratios of 16O 18O and 16 2O TD intensities to the sum of three TD intensities at the same peak, are: R( 16O 18O) = 0.45 and R( 16O 2) = 0.05 for T p = 200 K, and R( 16O 18O) = 0.20 and R( 16O 2) = 0.20 for T p = 690−750 K. A plausible exchange mechanism is discussed in terms of interactions of 18O 2 adsorbates with low-coordinated 16O − ions and 16O 2 2- peroxy defects on MgO surfaces.
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