Oxygen-isotope exchange between CO adsorbate and MgO surfaces

By using thermal-desorption gas analysis, oxygen-isotope exchange in the temperature range 100--800 K is found between C18O gas and MgO powders baked in high vacuum. The exchange ratio (=the ratio of C16O desorption intensity to the sum of C16O and C18O desorption intensities after C18O adsorption) is more than 0.6 after CO exposures less than about 103 L (1 L==1x10-6 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 models. 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 O 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