The actual Co(10-12) surface structure and CO activation

The hexagonal close-packed Co(10-12) surface has two atomic arrangements. The Co(10-12)-A termination, featuring sevenfold, eightfold, and elevenfold coordinated Co atoms, has been predominantly adopted in previous studies, based on a low-energy electron diffraction (LEED) intensity/voltage study of a Co(10-12) single crystal. Herein, we show that subsurface oxygen species (O_sub) in Co(10-12) single crystal, hardly removed by the routine method of ion sputtering followed by annealing, affects the LEED spot intensity profile. Via a strategy of H₂-induced segregation of O_sub followed by ion sputtering and annealing, a O_sub-free clean Co(10-12) surface is prepared and exhibits the Co(10-12)-B termination with sevenfold, ninefold, and tenfold coordinated Co atoms. Direct CO activation is much favored than H-assisted CO activation on Co(10-12)-B surface, different from previous reports on Co(10-12)-A surface. Thus, relevant fundamental understanding of Fischer-Tropsch synthesis on Co catalysts previously acquired using Co(10-12)-A surface needs to be reconsidered using Co(10-12)-B surface.