Structural Differentiation of the Reactivity of Alcohols with Active Oxygen on Au(110)

A wide range of oxidative coupling reactions occur under mild conditions on gold, all of which are activated by atomically adsorbed oxygen (O_ads). We have examined the reactivity of methanol and ethanol for self-coupling with well-ordered O-covered Au(110) surface structures, characterized by scanning tunneling microscopy (STM). Zig-zag O chains along the reconstructed (1 × 2) surface are observed up to a coverage of 0.25 Ml O that evolve into double row structures filling all threefold hollow sites along the topmost rows of gold atoms up to 0.5 Ml. Surface roughening occurs above 0.5 Ml O. Below 0.08 Ml O, both alcohols exhibit 100% selectivity for self-coupling to their corresponding ester. This adsorbed O consumed by abstraction of the alcoholic hydrogen, to form the adsorbed alkoxide. Under these conditions further C–H bond of both the adsorbed alkoxy and hemiacetal alcoholate intermediates necessary to form the ester predominates over reactions assisted by O_ads. Above ~0.08 Ml the alcohols react very differently. Methanol is not activated by additional adsorbed O, whereas ethanol reacts readily up to a coverage of 0.35 Ml O. We attribute this differentiation to both a change in the energetics of the O bonding to the surface and differences in the energetics of reaction of the two alcohols to form adsorbed alkoxides.