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

碱土氧化物细粒表面与CO气体之间的氧交换机理研究

基本信息

批准号：
01550017
负责人：
HUZIMURA Ryoiytiro
金额：
$ 0.96万
依托单位：
Nara University of Education
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1989
资助国家：
日本
起止时间：
1989 至 1990
项目状态：
已结题

项目摘要

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.

在本研究中，我们发现CO2气体<16><18>在碱土金属氧化物粉末上吸附后发生热脱附（TD），并对氧交换机理进行了如下解释.对于MgO，在200、400和600 K温度下发生CO-TD，交换比（R= CO_2<16>峰强度与CO_2和CO_2峰强度之和<16>的比值<18>）即使在低温峰也为0.65，在室温以上峰为0.8-0.9。R值随CO_2暴露量的增加而降低<18>，在高真空中高温焙烧的氧化物的R值降低程度更大。电子显微镜观察表明这种氧化物具有更复杂的表面结构.从头计算MO假设MgO表面团簇的计算表明，一个平面表面上的单点空位是不活跃的交换。Mg<111>（111）和O（111）面之间的台阶界面上的氧空位可以形成CO_3型桥连中间体来诱导交换. CaO和SrO的R值接近MgO的R值，但TD曲线略有不同，为了比较，我们研究了TiO_2和ZnO的TD，发现R值与RO峰一致<approximately equal>，而NiO的TD值与MgO的R值几乎相同.从以上结果我们可以得出结论，氧同位素交换不是通过电子过程，而是通过氧化物表面缺陷处的结构变化进行的。