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Nanoporous gold,a dilute alloy of Ag in Au,displays a remarkable activity and selectivity to the selective oxidative coupling of methanol to methyl formate.Previous experimental and theoretical studies conjecture residual Ag activates molecular oxygen and Au and Au/Ag mixed sites promote the oxygen-assisted catalytic coupling of methanol.However,based on the fact that O2 dissociation has to surmount high energy barriers and oxygen atom is a strong oxidant and can oxidize methanol to CO2,which is a by-product,the mechanism of O-assisted selective coupling of methanol is doubtful.We proposed the new mechanism of OH-assisted selective coupling of methanol to form the ester,on molecular-oxygen-covered AuAg(111)surface using density functional theory calculations.The first step in the process is dissociation of the O-H bond in methanol,which transfers the proton to adsorbed oxygen molecule to form OOH.Then the OOH reacts with another methanol to form two OH and CH3O.And the barrier of two steps is much lower than that of O2 dissociation.The produced OH induces the ensuing β-H elimination in CH3O to form CH2O with the barrier of 1.08eV.Subsequent coupling of CH3O and CH2O has low barrier in calculation.So β-H elimination is the rate-limiting step for the overall reaction,which is consistent with the experimental conclusion.Meanwhile OH is a weak oxidant compared to O atom and can reduce the by-product of CO2.The newly proposed OH-assisted selective coupling of methanol can rationalize the remarkable activity and selectivity of nanoporous gold.