Fischer-Tropsch Synthesis: Influence of Mn on the Carburization Rates and Activities of Fe-Based Catalysts by TPR-EXAFS/XANES and Catalyst Testing

Fe-based catalysts containing different amounts of Mn were tested for Fischer-Tropsch synthesis using a stirred tank reactor at 270 degrees C, 1.21 MPa, and H-2:CO = 0.7. Catalyst activation by carburization with 10% CO/He was followed by Temperature Programmed Reduction/X-ray Absorption Spectroscopy (TPR-EXAFS/XANES) from room temperature to 300 degrees C. gamma-Fe2O3 was converted into iron carbides, whereas MnOx was reduced to oxygen deficient MnO. Mn hindered Fe carburization, such that the carburized catalyst displayed higher Fe3O4 content than the catalyst without Mn. EXAFS fitting indicates that the carburized catalyst contained a mixture of Hagg carbide, Fe3O4, and Mn oxides. Increasing Mn content led to higher CH4 and light product selectivities, and lower light olefin selectivities. Higher and stable conversions were obtained with a catalyst containing an almost equimolar Fe/Mn ratio relative to the catalyst without Mn. Selectivity trends are attributed to the higher WGS rates observed on the FeMn catalysts, consistent with the structural differences observed.