Characterization of working iron Fischer-Tropsch catalysts using quantitative diffraction methods

This study presents the results of the ex-situ characterization of working iron Fischer-Tropsch synthesis (F-TS) catalysts, reacted hundreds of hours at elevated pressures, using a new quantitative x-ray diffraction analytical methodology. Compositions, iron phase structures, and phase particle morphologies were determined and correlated with the observed reaction kinetics. Conclusions were drawn about the character of each catalyst in its most and least active state.; The identity of the active phase(s) in the Fe F-TS catalyst has been vigorously debated for more than 45 years. The highly-reduced catalyst, used to convert coal-derived syngas to hydrocarbon products, is thought to form a mixture of oxides, metal, and carbides upon pretreatment and reaction. Commonly, Soxhlet extraction is used to effect catalyst-product slurry separation; however, the extraction process could be producing irreversible changes in the catalyst, contributing to the conflicting results in the literature.; X-ray diffraction doesn't require analyte-matrix separation before analysis, and can detect trace phases down to 300 ppm/2 nm; thus, working catalyst slurries could be characterized as-sampled. Data were quantitatively interpreted employing first principles methods, including the Rietveld polycrystalline structure method. Pretreated catalysts and pure phases were examined experimentally and modeled to explore specific behavior under x-rays. Then, the working catalyst slurries were quantitatively characterized. Empirical quantitation factors were calculated from experimental data or single crystal parameters, then validated using the Rietveld method results.; In the most active form, after pretreatment in H ₂ or in CO at P_ambient, well-preserved working catalysts contained significant amounts of Fe₇C₃ with trace α-Fe, once reaction had commenced at elevated pressure. Amounts of Fe₃O ₄ were constant and small, with carbide d_pavg < 15 nm. Small amounts of Fe₇C₃ were found in unreacted catalyst pretreated in CO at elevated pressures.; In the least active form, well-preserved working catalysts contained Fe₅C₂ amounts >65 wt%, regardless of pretreatment gas and pressure, with all d_pavg 18 nm. ε ′-Fe_2.2C carbide was found to probably consist of an {lcub}Fe₅C₂/Fe_xO/ε-Fe₃C{rcub} mixture. Fe₅C₂ carbide exhibited wide variations in diffraction pattern which could be correlated with sample handling events, changes in process conditions, or d_pavg.