Rational design of silica-supported platinum-promoted iron Fischer-Tropsch synthesis catalysts based on activity-structure relationships

Silica-supported iron catalysts (Fe/SiO₂, FePt/SiO₂ and FePtK/SiO₂) were prepared using an advanced non-aqueous (acetone) evaporative deposition technique. This preparation leads to a catalyst with high extents of reduction, moderately-high dispersions, high CO conversions, and high C₂₊ hydrocarbons productivities comparable to commercial-grade unsupported, precipitated iron catalysts of high activity and selectivity. Catalyst activities were tested following an L18 orthogonal array statistical design approach. Primary independent variables affecting catalysts activity were promoter type, pretreatment gas composition (H₂, H₂/CO, or CO), pretreatment temperature (250, 280, or 320°C), and reaction temperature (250 or 265°C). Activity was found to increase in the order Fe/SiO ₂, FePt/SiO₂ and FePtK/SiO₂. For a given catalyst composition, activity increases through a maximum with increasing pretreatment temperature and increasing time. Steady state catalytic activity is positively correlated with increasing bulk chi-carbide content (from Mössbauer spectroscopy) for silica-supported Fe and FePt catalysts but not for FePtK.; A coordinated characterization of both the bulk and surface carbonaceous species was conducted with high pressure in situ Mössbauer spectroscopy and TPSR-MS. A correlation between the amount of reactive α-carbon (C _α) and initial catalytic activity was established based on this TPSR work. It is concluded that the method of Li et al.'s [1] which actually measured irreversibly chemisorbed CO site density does not provide a quantitative measure of active site density for FTS on silica supported iron catalyst. Nevertheless, this method was adapted in this study for quantitative measurement of irreversible CO site density on supported iron catalysts.; Finally, a more comprehensive model was proposed for iron phase transformations during pretreatment, FTS and post-reaction passivation/oxidation. This model can resolve a number of discrepancies and explain the observed phenomena in some relevant literature.