tA series of Fe-Ti mixed oxide model catalysts containing different Ti/Fe ratios were synthesized andapplied as catalysts for the High Temperature Fischer-Tropsch reaction (HTFTS). XRD, H2-TPR and in situMössbauer and XAFS spectroscopy were applied to evaluate the role of Ti on the physical and chemicalproperties of Fe within the mixed metal oxide. It was observed that the Ti/Fe ratio determines the relativeamounts of hematite, pseudobrookite, and anatase in the starting materials. The interplay between thesephases is responsible for the HTFTS catalytic performance.Our results demonstrate that the presence of pseudobrookite: i) enhances the dispersion of iron; ii)mediates and controls the reduction and carburization degree during the transformation of Fe (III) speciesto carbides upon activation, and iii) increases the stability under HTFTS conditions by minimizing the re-oxidation of iron carbides. Highest catalytic activity and stability is achieved for the material with Ti/Feratio of 1/2.1.@en

The structure and elementary composition of various commercial Fe-based MOFs used as precursors for Fischer-Tropsch synthesis (FTS) catalysts have a large influence on the high-temperature FTS activity and selectivity of the resulting Fe on carbon composites. The selected Fe-MOF topologies (MIL-68, MIL-88A, MIL-100, MIL-101, MIL-127, and Fe-BTC) differ from each other in terms of porosity, surface area, Fe and heteroatom content, crystal density and thermal stability. They are re-engineered towards FTS catalysts by means of simple pyrolysis at 500 °C under a N₂ atmosphere and afterwards characterized in terms of porosity, crystallite phase, bulk and surface Fe content, Fe nanoparticle size and oxidation state. We discovered that the Fe loading (36-46 wt%) and nanoparticle size (3.6-6.8 nm) of the obtained catalysts are directly related to the elementary composition and porosity of the initial MOFs. Furthermore, the carbonization leads to similar surface areas for the C matrix (S_BET between 570 and 670 m² g^-1), whereas the pore width distribution is completely different for the various MOFs. The high catalytic performance (FTY in the range of 1.9-4.6 × 10^-4 mol_CO g_Fe ^-1 s^-1) of the resulting materials could be correlated to the Fe particle size and corresponding surface area, and only minor deactivation was found for the N-containing catalysts. Elemental analysis of the catalysts containing deliberately added promoters and inherent impurities from the commercial MOFs revealed the subtle interplay between Fe particle size and complex catalyst composition in order to obtain high activity and stability next to a low CH₄ selectivity.

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In this combined in situ XAFS, DRIFTS, and Mössbauer study, we elucidate the changes in structural, electronic, and local environments of Fe during pyrolysis of the metal organic framework Fe-BTC toward highly active and stable Fischer-Tropsch synthesis (FTS) catalysts (Fe@C). Fe-BTC framework decomposition is characterized by decarboxylation of its trimesic acid linker, generating a carbon matrix around Fe nanoparticles. Pyrolysis of Fe-BTC at 400 °C (Fe@C-400) favors the formation of highly dispersed epsilon carbides (?′-Fe_2.2C, d_p = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly Hägg carbides are formed (?-Fe₅C₂, d_p = 6.0 nm). Extensive carburization and sintering occur above these temperatures, as at 900 °C the predominant phase is cementite (?-Fe₃C, d_p = 28.4 nm). Thus, the loading, average particle size, and degree of carburization of Fe@C catalysts can be tuned by varying the pyrolysis temperature. Performance testing in high-temperature FTS (HT-FTS) showed that the initial turnover frequency (TOF) of Fe@C catalysts does not change significantly for pyrolysis temperatures up to 600 °C. However, methane formation is minimized when higher pyrolysis temperatures are applied. The material pyrolyzed at 900 °C showed longer induction periods and did not reach steady state conversion under the conditions studied. None of the catalysts showed deactivation during 80 h time on stream, while maintaining high Fe time yield (FTY) in the range of 0.19-0.38 mmol_CO g_Fe ^-1 s^-1, confirming the outstanding activity and stability of this family of Fe-based FTS catalysts.

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