On the kinetics and mechanism of Fischer–Tropsch synthesis on a highly active iron catalyst supported on silica-stabilized alumina

• Overall kinetics is similar for supported and unsupported Fe catalysts.
• CO follows a direct dissociation pathway on supported Fe catalysts.
• The hydrogenation steps are rate determining resulting in a high pH 2 dependency.
• Carbon on the surface is the most abundant surface intermediate.

The kinetics of a supported iron Fischer–Tropsch (FT) catalysts were investigated and a physically meaningful model that fits the data very well is proposed. Kinetic data (reported herein) were obtained at 250 °C and 20 atm in a fixed bed reactor at a variety of PH2PH2 and PCO. Measured PH2PH2 and PCO power law dependencies were found to be in the same range as those for unsupported Fe FT catalysts previously reported. The kinetic models in this study were tested using a statistical lack-of-fit test. Eight, two-parameter Langmuir–Hinshelwood rate expressions based on various mechanistic routes and assumptions were derived and tested, but all gave relatively poor fits to the data. An adjustment of the PH2PH2 dependency of the derived expressions to the 0.875 power resulted in three reasonable semi-empirical models, one of which fit the data extremely well. This approach also allowed us to determine the best function of PCO dependency. The results suggest that supported Fe FT catalysts follow a direct CO dissociation pathway, that carbon is one of the most abundant species on the surface of the catalyst, and that the hydrogenation of either C* or CH* is the rate-determining step.

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