Mn-Promoted Co/TiO ₂ Catalysts: Quantitative Analysis of Cobalt Polymorphs and Stacking Faults and Its Effect on Fischer-Tropsch Synthesis Performance

The transition to net-zero emissions hinges on circular economy strategies that valorize waste and enhance resource efficiency. Among X-to-liquid (XTL) technologies, the Fischer-Tropsch (FT) process stands out for converting biomass, waste, and CO₂ into hydrocarbons and chemicals, especially when powered by renewable hydrogen. Cobalt-based catalysts are preferred in FT synthesis due to their efficiency and CO₂ tolerance, yet their catalytic performance is closely tied to their polymorphic structuresface-centered cubic (FCC), hexagonal close-packed (HCP), and stacking-faulted intergrowths thereof. HCP cobalt has been shown to exhibit high activity and selectivity for higher hydrocarbons and oxygenates, particularly when transformed into cobalt carbide (Co₂C), which forms more readily at low H₂/CO ratios. This study presents a quantitative analysis of cobalt polymorphs and stacking faults in Mn-promoted Co/TiO₂ FT catalysts from in situ powder X-ray diffraction (XRD) data and X-ray Diffraction Computed Tomography (XRD-CT) data from spent catalysts in order to obtain a more complete correlation of structural features with catalytic performance. By modeling stacking fault probabilities using supercell simulations, the proportion of faulted FCC and HCP domains was determined across varying Mn loadings (0-5%). Increased Mn loading was found to decrease stacking faults in the FCC phase while increasing them in HCP, promoting the formation of HCP domains and ultimately Co₂C under reaction conditions. Notably, the 3% Mn-loaded sample showed a marked rise in HCP content and Co₂C formation, correlating with the highest observed alcohol and olefin selectivity. These findings highlight a critical structure-function relationship: Mn facilitates a transformation from FCC to HCP and then to Co₂C, this final transition driven by similar stacking sequences and metal-support interactions. The findings show that Mn promotion not only stabilizes smaller Co particles and enhances its dispersion, but also modulates the distribution of Co polymorphs and stacking faults, leading to altered catalytic behavior. This highlights the importance of stacking fault characterization for optimizing FT catalyst design and performance, and suggests pathways to more efficient and selective carbon-neutral fuel production through engineered polymorphic and interfacial structures.