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

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 degrees 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 P-H2 increases with increasing temperature while the rate coefficient for P-CO decreases with temperature going from positive order (+0.3) at 210 degrees C to negative order (-0.6) at 240 degrees 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, K-CO*P-CO which, since the denominator is squared in LH expressions, can allow for an overall negative order P-CO dependence. However, since K-CO is the equilibrium constant for the adsorption of CO, its value decreases with increasing temperature which causes the overall P-CO 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)*P-CO 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. (C) 2016 Elsevier B.V. All rights reserved.