Spin Trapping of Au-H Intermediate in the Alcohol Oxidation by Supported and Unsupported Gold Catalysts
Spin Trapping of Au-H Intermediate in the Alcohol Oxidation by Supported and Unsupported Gold Catalysts
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DOI：
10.1021/ja809883c
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发表时间：
2009-05-27
影响因子：
15
通讯作者：
Chechik, Victor
中科院分区：
文献类型：
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作者：
Conte, Marco;Miyamura, Hiroyuki;Chechik, Victor
采用电子顺磁共振（EPR）谱和自旋捕获技术研究了金催化剂上醇氧化反应的机理。仲醇与负载型和非负载型金催化剂（例如，Au/CeO 2、聚合物嵌固的Au纳米颗粒、PPh 3保护的Au纳米颗粒）在自旋陷阱存在下导致氢自旋加合物的形成。利用同位素标记，我们证实了自旋加合物中的氢来源于醇分子中C-H键的断裂。氢自旋加合物的形成最有可能是由于自旋陷阱从Au表面提取氢。因此，这些结果强烈表明在酒精过程中中间形成Au-H物质。这进一步证实了进行金催化的醇氧化在没有氧气的情况下，与氮氧化物作为氢提取。支撑物（例如，金属氧化物）可以活化氧并作为从金表面的H提取剂，因此导致活性的更快恢复。在醇氧化过程中也观察到过氧自由基，与自由基自氧化机制一致。然而，这种机制可能是一种轻微的副反应，不会导致形成可观量的氧化产物。
Electron paramagnetic resonance (EPR) spectroscopy and spin trapping were used to explore the mechanism of alcohol oxidation over gold catalysts. Reaction of secondary alcohols with supported and unsupported gold catalysts (e.g., Au/CeO2, polymer-incarcerated Au nanoparticles, PPh3-protected Au nanoparticles) in the presence of spin traps led to the formation of a hydrogen spin adduct. Using isotope labeling, we confirmed that the hydrogen in the spin adduct originates from the cleavage of the C-H bond in the alcohol molecule. The formation of the hydrogen spin adduct most likely results from the abstracting of hydrogen from the Au surface by a spin trap. These results thus strongly suggest intermediate formation Au-H species during alcohol. This was further confirmed by carrying out gold-catalyzed alcohol oxidation in the absence of oxygen, with nitroxides as hydrogen abstractors. The support (e.g., metal oxides) can activate oxygen and act as an H abstractor from the gold surface and hence lead to a faster recovery of the activity. Peroxyl radicals were also observed during alcohol oxidation, consistent with a free-radical autoxidation mechanism. However, this mechanism is likely to be a minor side reaction, which does not lead to the formation of an appreciable amount of oxidation products.