Step Site-Specific Semihydrogenation of Acetylene on the Au Surface.

Supported Au catalysts are highly selective and size-sensitive in catalytic hydrogenation of alkynes under mild conditions. Using thermal-programmed desorption and density functional theory calculations, we study the hydrogenation reactions of C 2 hydrocarbons with atomic H and clarify the site-specific selective hydrogenation of C 2 H 2 on Au(997) at low temperatures. On atomic H(a) covered Au(997), hydrogenation of C 2 H 2 goes with 100% selectivity to C 2 H 4 at steps, yet no hydrogenation occurs at terraces; adsorbed C 2 H 4 on neither steps nor terraces reacts with H(a). DFT calculations suggest that the increased adsorption free energies and appropriate reaction barriers of C 2 species at steps lead to the step-site specific semihydrogenation of C 2 H 2 . These results elucidate the elementary surface reactions between C 2 hydrocarbons and atomic H on Au surfaces at the molecular level and significantly deepen the fundamental understanding of the unique selectivity of Au catalysts.