Kinetics of Fischer-Tropsch synthesis on supported cobalt: Effect of temperature on CO and H2 partial pressure dependencies

• Pco dependency of FT rate decreases from +0.31 to −0.64 as temp increases.
• Parallel H-assisted pathways leads to activated denominator term.
• Model explains observed presence of atomic carbon on the catalyst surface.
• “Lack of fit” P parameter effectively differentiates between kinetic mechanisms.

Kinetic data were measured for Fischer-Tropsch Synthesis (FTS) on a cobalt catalyst supported on silica-modified alumina at various partial pressures of CO and H2 and at four different temperatures (210, 220, 230, and 240 °C). The data were sufficient to determine power law rate expressions at each of the four temperatures which indicate that the dependence of rate on PH2 increases with increasing temperature while the rate coefficient for PCO decreases with temperature going from positive order (+0.3) at 210 °C to negative order (−0.6) at 240 °C. Several mechanisms were explored and Langmuir–Hinshelwood (LH) rate models derived in an attempt to explain the data trends. Traditional FT rate expressions have a denominator term, KCO*PCO which, since the denominator is squared in LH expressions, can allow for an overall negative order PCO dependence. However, since KCO is the equilibrium constant for the adsorption of CO, its value decreases with increasing temperature which causes the overall PCO dependence to become more positive with increasing temperature instead of more negative. In this work we propose a mechanism based on parallel hydrogen-assisted mechanistic pathways that leads to a LH model with the denominator term, k’CO*PCO where k’CO is effectively an activated rate constant instead of an equilibrium constant and therefore increases with increasing temperature instead of decreasing. This model, which fits the data extremely well, also explains the presence of atomic carbon on the surface of the catalyst.

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