De l’évolution de l’orthodontie

Abstract

The reaction mechanism of selective oxidation of ethanol on Au(111) covered with atomic oxygen was described employing density functional theory calculations. The first step, dissociation of the O–H bond in ethanol, has lower barrier for transfer of the H to adsorbed oxygen or surface hydroxyl than to gold with a barrier of 0.20 or 0.17 eV, respectively. The two lower activation energies for the β-H elimination of ethoxyl, 0.29 and 0.27 eV, come from transfer of H atom to surface O atom or another ethoxyl. Ethyl acetate is formed through the β-H elimination of ethoxy hemiacetal. The formation of acetic acid is not facile through the reaction between the surface hydroxyl and acetaldehyde or between the surface oxygen and acetaldehyde at low temperature due to a high activation barrier. Except for surface oxygen and bidentate acetate, all other surface species have low diffusion barriers, suggesting that rearrangement and movement of these species from the preferred adsorption sites to the ideal configurations for reactions are facile.采用密度泛函理论研究了吸附有O原子的Au(111)表面上乙醇选择性氧化的反应机理. 反应结果表明,除O原子和中间产物二齿醋酸根(CH3CHOO)外,其他中间产物在Au(111)表面扩散能垒均较低,不会对反应速控步骤的确定造成影响. 乙醇羟基氧化脱氢为反应的第一步骤,当氧化剂为吸附态的O原子或者为OH基时,反应活化能分别为0.20和0.17eV. 氧化产物乙氧基(CH3CH2O)进一步氧化脱氢生成乙醛则需要表面吸附的O原子或另一表面吸附的OH基的参与,所需活化能为0.29或0.27eV. 同时,乙醛易与表面吸附的乙氧基反应生成乙氧基半缩醛(CH3CHOOC2H5),其可进一步与O原子作用,脱氢形成乙酸乙酯. 此外,在乙醛深度氧化成酸的过程中需要克服较高的反应能垒,因而在表面反应温度较低时无法进行,这与实验结果相符.