The role of Manganese in CoMnOx Catalysts for Selective Fischer-Tropsch Synthesis

The origin of the high selectivity of cobalt-manganese oxide (CoMnOx) catalysts in the Fischer−Tropsch synthesis (FTs) reaction towards long-chain hydrocarbon products was investigated using model systems of CoMnOx in the form of crystalline nanoparticles and amorphous thin films where Co and Mn are intimately mixed rather than separated in two phases. Using ambient pressure X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy, the chemical structure of the catalyst and adsorbed species were determined under reaction conditions. We found that the catalytically active phase contains an outer layer enriched in metallic Co relative to the bulk. Molecular CO adsorbs on Co0 sites, where it dissociates by reaction with H2 to form cobalt carbide and CHx species. The concentration of CHx increases rapidly with exposure to CO/H2 syngas on the CoMnOx catalyst, while no such increase was observed in the absence of Mn. Density Functional Theory (DFT) simulations indicate that MnO acts as a reservoir of H atoms bound to the basic O sites, which makes it less accessible to CHx moieties, thus hindering chain termination to form short chain hydrocarbons. In contrast, the increasing concentration of CHx moieties helps chain growth.