Towards understanding the lower CH4 selectivity of HCP-Co than FCC-Co in Fischer–Tropsch synthesis

Abstract

In Fischer–Tropsch synthesis (FTS), the cobalt catalyst has higher C₅₊ and lower CH₄ selectivity in the hcp phase than in the fcc phase. However, a detailed explanation of the intrinsic mechanism is still missing. The underlying reason was explored combining density functional theory, Wulff construction, and a particle-level descriptor based on the slab model of surfaces that are prevalent in the Wulff shape to provide single-particle level understanding. Using a particle-level indicator of the reaction rates, we have shown that it is more difficult to form CH₄ on hcp-Co than on fcc-Co, due to the larger effective barrier difference of CH₄ formation and C–C coupling on hcp-Co particles, which leads to the lower CH₄ selectivity of hcp-Co in FTS. Among the exposed facets of fcc-Co, the (311) surface plays a pivotal role in promoting CH₄ formation. The reduction of CH₄ selectivity in cobalt-based FTS is achievable through phase engineering of Co from fcc to hcp or by tuning the temperature and size of the particles.