Abstract
Cobalt is an efficient catalyst for Fischer-Tropsch synthesis (FTS) of hydrocarbons from syngas (CO + H(2)) 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 H(2) dissociation on Co(0) sites, making them less available to react with CH(x) intermediates, thus hindering chain termination reactions, which promotes the formation of long-chain hydrocarbons.