碘乙醇在Ni(100)表面的吸附和热分解-碳氢化合物氧化的中间产物：羟乙基和氧金属环

利用热脱附(TPD)实验和X射线光电子能谱(XPS)研究了碘乙醇在Ni(100)表面的吸附和热反应过程. 实验结果表明碘乙醇在100 K时以两种分子的形式吸附在Ni(100)的表面, 即: 以碘原子端吸附在表面或以碘原子端和羟基端同时吸附在表面. 由于两种吸附形式的分子的一致分解和吸附分子的不均匀性, 在140 K引起了较复杂的化学反应, 伴有少量的乙烯和水产生. 碘乙醇在150 K经过C—I键断裂, 有80%碘乙醇生成—O(H)CH2CH2—中间产物, 20%的碘乙醇生成羟乙基中间产物. 羟乙基在160 K的转化过程中包括两个互相竞争的化学反应: 与表面的氢原子进行还原反应生成乙醇, 或失去一个β-H原子生成表面乙烯醇. 另外, 在相同的温度下—O(H)CH2CH2—中间产物经过脱氢反应产生—OCH2CH2—氧金属环. 羟乙基和氧金属环都会发生异构, 分别在210 K和250 K生成乙醛, 这些乙醛一部分从表面脱出, 其余的部分发生分解反应产生氢气、水和一氧化碳. 在实验基础上, 进一步探讨了这种化学过程在催化中的作用和指导意义.

Adsorption and Thermal Decomposition of 2-iodoethanol on Ni(100)——Intermediates of Oxidation of Hydrocarbons: Hydroxyalkyls and Oxametallacycles

Based on the study of the adsorption and thermal reactions of 2-iodoethanol on clean Ni(100) surface, further research and discussion were conducted on the results of experimental data, and the mechanism of chemical reaction was analyzed in detail. Two potential intermediates (2-hydroxyethyl and oxametallacycle surface species) in catalytic oxidation hydrocarbons were studied. 2-iodoethanol adsorbed molecularly on Ni(100) at 100 K starts chemical reaction around 140 K and produces small amounts of ethylene and water, due to the concerted decomposition or disproportionation of the adsorbed molecular species. Around 150 K with an initial C-I bond scission, 80% of 2-iodoethanol decompose to form -O(H)CH2CH2-, while 20% of 2-iodoethanol transform to 2-hydroxyethyl intermediates. The subsequent reaction of the 2-hydroxyethyl species around 160 K concerns two competing reactions: a reductive elimination with surface hydrogen to yield ethanol, and a β-H elimination to from surface vinyl alcohol. At the same temperature, the -O(H)CH2CH2- intermediate dehydrogenates to a -OCH2CH2- oxametallacycle species. Both 2-hydroxyethyl and oxametallacycle species tautomerize to acetaldehyde, around 210 K and above 250 K, respectively. Some of that acetaldehyde desorbs while the rest decomposes to hydrogen and carbon monoxide. The implications of this chemical process to catalysis are discussed.