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

在本研究中，我们发现CO^&lt；16&gt；在碱土氧化物粉末上吸附CO^&lt；18&gt；气体后发生热脱附(TD)，氧交换机理解释如下1。对于氧化镁，CO-Td发生在200、400和600K，交换比(R=CO^lt；16&gt；峰强度与CO^lt；16&gt；和CO^&lt；18&gt；；)即使在低温峰也达到0.65，在室温以上的峰达到0.8-0.9。R值随着CO^&lt；18&gt；曝光量的增加而减小，更大程度上是在高真空中高温焙烧的氧化物。电子显微镜观察表明，这种氧化物的表面结构更加复杂。假设氧化镁表面团簇的从头算表明，平面表面上的单点空位对交换是无效的。在Mg(111)和O(111)表面之间的&lt；111&gt；定向台阶界面上的氧空位可以提供一种途径来形成CO_3型桥联中间体来诱导交换。CaO和SRO的R值接近于MgO的R值，但Td剖面略有不同。为了进行比较，我们研究了二氧化钛和氧化锌的热释光，发现了R与lt；近似相等的峰，而从NiO中观察到的热释光的R与MgO的值几乎相同。根据以上结果，我们得出结论，氧-同位素交换不是通过电子过程进行的，而是通过氧化物表面缺陷处的结构变化进行的。

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.