Theoretical Study of O-Assisted Selective Coupling of Methanol on Au(111)

We report the first systematic theoretical study of the oxidative self-coupling of methanol to form the ester, methylformate, on atomic-oxygen-covered Au( 111) using density functional theory calculations. The first step in the process-dissociation of the O-H bond in methanol-has a lower barrier for transfer of the proton to adsorbed oxygen than for transfer of H to gold, consistent with experimental observations that O is necessary to initiate the reaction. The computed barrier for formation of methoxy (CH3O) and OH is 0.41 eV, compared with 1.58 eV calculated for the transfer of H to the clean Au surface. Several different pathways for the ensuing beta-H elimination in CH3O(ads) to form formaldehyde have been considered, namely, attack by adsorbed O, OH, or a second CH3O, and transfer to the Au metal. Methoxy attacked by surface oxygen has the lowest calculated barrier, 0.49 eV, and leads to adsorbed H2C=O and OH. Subsequent coupling of methoxy and formaldehyde has no apparent barrier in the calculation, consistent with the experimental conclusion that beta-H elimination is the rate-limiting step for the overall reaction. With the exception of surface oxygen, all other surface species have low diffusion barriers, suggesting that rearrangement and movement of these species from the preferred adsorption sites to configurations necessary for reactions occur readily, thus contributing to the activity for coupling on gold.