Fischer–Tropsch synthesis: Kinetics and water effect study over 25%Co/Al2O3 catalysts
•Determining the kinetic water effect on cobalt is important but quite challenge.•Kinetic and water co-feeding approaches were used to study the kinetic water effect.•Experiments with Co/Al2O3 catalysts were conducted during periods of stable catalytic activity.•A positive kinetic water effect occurs on stable (e.g., aged) metallic Co particles.•The CAER model gave a good fit to the kinetic data.
The kinetics of Fischer–Tropsch synthesis (FTS) over a 25%Co/Al2O3 catalyst was studied using a 1-L continuously stirred tank reactor (CSTR) under the conditions of 205–230 °C, 1.4–2.5 MPa, H2/CO = 1.0–2.5 and 3–16 NL/g-cat/h (XCO = 7–54%). Thirty-one sets of kinetic data collected at 220 °C with a low extent of deactivation were used for kinetic parameter regression. The CAER empirical kinetic model (rFT=kPCOaPH2b/(1+mPH2O/PH2)) was employed to study the kinetic effect of water. A positive kinetic water effect was first evidenced using the kinetic approach, consistent with the results of the effect of co-fed water on cobalt FTS in this work and the literature (e.g. Loegdberg et al., 2011 ) for Co/Al2O3 catalysts. The current kinetic results are based on kinetic data taken following an initial catalyst induction period, where the CO conversion had stabilized. These data are different from our earlier investigations where reversible oxidation of small cobalt crystallites and/or catalyst support effects likely impacted the cobalt site densities, resulting in a negative water effect. Thus, in this study, we decoupled the effect of reversible oxidation from the kinetics, so that the effect of water on stable (i.e., presumably larger) metallic cobalt particles could be assessed.In this study, an additional eleven classical FT kinetic models for Co catalysts were tested using the kinetic data. Five of them were also found to adequately describe the kinetic data, including two mechanistic models developed based on carbide mechanisms. The CAER model containing a water effect term and the mechanistic model of Botes et al. (2009)  yielded comparable reaction orders for the partial pressures of H2 and CO, resulted in a better fit of the kinetic data.
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Journal: Catalysis Today - Volume 228, 1 June 2014, Pages 158–166