Fischer-Tropsch synthesis (FTS) is an industrial catalytic process that converts a mixture of CO and hydrogen into long-chain hydrocarbons. These products are used as clean transportation fuels and chemical building blocks. The solid catalysts used in the process are complex multi-component systems. Therefore, unambiguously determining the catalytically active phase under reaction conditions remains challenging and thus a topic of debate. The active phase in cobalt-based FTS, including the reaction pathways it catalyzes, has been of industrial and academic interest for many years. It provides direct ways to control the output of the process. The delineation between an active and inactive phase is often unclear, as different phases (i.e., cobalt oxide, carbide, and metal) have different catalytic behavior. This review focuses on cobalt-based FTS materials, with a special focus on the industrially applied cobalt/TiO2 system. The various cobalt phases are reviewed and discussed with respect to the most recent literature
