Spin trapping of Au-H intermediate in the alcohol oxidation by supported and unsupported gold catalysts

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/CeO(2), polymer-incarcerated Au nanoparticles, PPh(3)-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 abstraction of hydrogen from the Au surface by a spin trap. These results thus strongly suggest intermediate formation of Au-H species during alcohol oxidation. The role of oxygen in this mechanism is to restore the catalytic activity rather than oxidize 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.