Engineering Cobalt-Based Thin Films for Fischer-Tropsch Synthesis.

We report on the synthesis of cobalt-based thin films via magnetron sputtering at varying conditions, including sputtering pressure, reactive gas composition, substrate temperature, and post-deposition annealing. The produced thin films exhibit diverse morphologies, crystalline phases, and oxidation states, such as metallic cobalt (Co0) and cobalt oxides (CoO and spinel-Co3O4). Two thin-film catalyst architectures, based on Co0 (TFCat1) and CoxOy (TFCat2), on Si/Al2O3/SiO2 support stacks are further developed as analogues of commercial powder-based Fischer-Tropsch synthesis (FTS) catalysts. The phase evolution in both of these model catalyst systems is investigated in situ under FTS-relevant reduction conditions using synchrotron-based grazing-incidence x-ray diffraction. Both catalysts (TFCat1 and TFCat2) were tested for their FTS catalytic performance at comparable operating conditions. A lined-out CO conversion level of ca. 4.6% and 4.9% was obtained for TFCat1 and TFCat2, respectively. These catalysts exhibited distinct steady-state product distributions: TFCat1 showed enhanced selectivity toward kerosene-range hydrocarbons, while TFCat2 produced a wider hydrocarbon distribution extending to C27. Moreover, Cobalt thin-film model catalyst (TFCat1) shows significantly reduced dewetting compared to its oxide counterpart (TFCat2) under FTS operating conditions. The well-defined and easy to tune cobalt-based thin films provide a robust foundation for the rational scale-up of next-generation catalysts.