Mechanism of the aerobic oxidation of methanol to formic acid on Au8-: A DFT study

The mechanism of the aerobic oxidation of methanol to formic acid catalyzed by Au8- has been systematically investigated using density functional theory with the M06 functional. The reaction pathways were examined by taking into account the full structural relaxation of the Au8-. Stepwise and concerted reaction mechanisms are proposed. The stepwise mechanism is initiated by the hydrogen abstraction of a methoxy species by a superoxo-like anion on the gold cluster, resulting in the formation of formaldehyde. Subsequently, the formaldehyde is activated by the hydroxyl group of a hydroperoxyl-like complex, leading to the formation of a hemiacetal intermediate. The formation of formic acid in the final step is achieved by hydrogen abstraction of the hemiacetal intermediate by atomic oxygen attached to the gold cluster. Our calculations indicate that the first step of the stepwise mechanism, that is, hydrogen abstraction of the methoxy species, is the rate-determining step. Another possible reaction pathway involving a single-step hydrogen abstraction, a concerted mechanism, is also discussed. This mechanism may also be responsible for the reasonable catalytic activity of aerobic oxidation of methanol on Au8- because of the low activation energy barrier. (C) 2012 Wiley Periodicals, Inc.