By using thermal-desorption gas analysis, oxygen-isotope exchange in the temperature range 100 -800 K is found between C ' 0 gas and MgO powders baked in high vacuum. The exchange ratio (=the ratio of C ' 0 desorption intensity to the sum of C ' 0 and C "0 desorption intensities after C "0 adsorption) is more than 0.6 after CO exposures less than about 10' L (1 L= -1X10 Torr s), above which the ratio goes down quickly. Since transmission-electron-microscopic observation suggests the importance of defective surfaces of ultrafine grains, the exchange process is investigated with use of an ab initio molecular-orbital calculation with several defective-cluster mode1s. The contribution to the exchange of simple point vacancies on a flat surface is ruled out, and a site having a combination of an oxygen vacancy and low-coordinated oxygen atoms is found to give a reasonable mechanism. Thereby two structural characteristics are derived for the exchange: (1) formation of a bridge-bonded intermediate and (2) subsequent insertion of the CO oxygen to the nearby 0 vacancy through isomerization of the intermediate. The cooperation of these processes can convert a CO multiple bond to a single bond for ready thermal exchange on d-orbital-free oxides.
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