The role of manganese in CoMnOx catalysts for selective long-chain hydrocarbon production via Fischer-Tropsch synthesis

Cobalt is an efficient catalyst for Fischer−Tropsch synthesis (FTS) of hydrocarbons from syngas (CO + H₂) with enhanced selectivity for long-chain hydrocarbons when promoted by Manganese. However, the molecular scale origin of the enhancement remains unclear. Here we present an experimental and theoretical study using model catalysts consisting of crystalline CoMnO_x nanoparticles and thin films, where Co and Mn are mixed at the sub-nm scale. Employing TEM and in-situ X-ray spectroscopies (XRD, APXPS, and XAS), we determine the catalyst’s atomic structure, chemical state, reactive species, and their evolution under FTS conditions. We show the concentration of CH_x, the key intermediates, increases rapidly on CoMnO_x, while no increase occurs without Mn. DFT simulations reveal that basic O sites in CoMnO_x bind hydrogen atoms resulting from H₂ dissociation on Co⁰ sites, making them less available to react with CH_x intermediates, thus hindering chain termination reactions, which promotes the formation of long-chain hydrocarbons.