Study of Oxygen-Exchange Mechanism Between Fine Grain Surfaces of Alkali Earth Oxides and CO Gas

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In this study we have found that Thermal Desorption (TD) of CO^<16> takes place after adsorption of CO^<18> gas on powders of alkali earth oxides and the oxygen exchange mechanism is explained as follows.1. For MgO, CO-TD occurs at temperatures 200, 400 and 600K and the exchange ratio (R=the ratio of the CO^<16> peak intensity to the sum of intensities of CO^<16> and CO^<18>) amounts to 0.65 even in the low temperature peak and 0.8-0.9 in the peaks above room temperature. The R values decrease with the increase of CO^<18> exposure, to a more extent for the oxide baked at higher temperatures in high vacuum. Electron microscopic observation suggests more complicated surface structures for this oxide.2. Ab initio MO calculations assuming MgO surface clusters suggests that a single point vacancy on a plane surface is inactive for the exchange. An oxygen vacancy on a <111>-directed step boundary between Mg(111) and O(111) surfaces can provide a way to form a CO_3-type bridged intermediate to induce the exchange.3. The R values of CaO and SrO are near to that of MgO although slight differences in TD profiles are found. For comparison we studied TD's from TiO_2 and ZnO in which peaks with R<approximately equal>O were found while we observed that from NiO with almost same magnitudes of R as MgO.4. From above results we conclude that the oxygen-isotope exchange takes place not through electronic processes but through structural changes at surface defects of the oxides.