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 (Osub) 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 Osub followed by ion sputtering and annealing, a Osub-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. Cobalt catalysts turn carbon monoxide into fuels, but their active surface structure has been uncertain. This study identifies an actual cobalt surface structure and shows it changes how carbon monoxide reacts.
Wang et al. (2026) studied this question.