Microkinetic modelling of the formation of C 1 and C 2 products in the Fischer–Tropsch synthesis over cobalt catalysts

Abstract

The heats of adsorption of different C 1 and C 2 molecules assumed to be present during the initial steps of the Fischer–Tropsch synthesis and activation energies for elementary steps envisioned to occur in the synthesis are calculated for Co by using the unity bond index-quadratic exponential potential (UBI-QEP) method. The preexponential factors for the elementary steps are calculated from transition-state theory, and the rate constants are calculated according to the Arrhenius equation. The activation barrier for hydrogenation of CO is found to be lower compared to hydrogen assisted dissociation of CO, which has a smaller activation barrier than direct dissociation of CO. The reaction steps with high activation barriers are eliminated. Based on this elimination two sets of elementary steps for formation of C 1 and C 2 alkenes and alkanes in the Fischer–Tropsch synthesis are established: one based on hydrogen assisted CO dissociation (carbide mechanism) and one based on CO hydrogenation (CO insertion mechanism). In addition, one mechanism of producing CO 2 from the water–gas shift reaction is proposed. The resulting mechanisms are combined and used in the microkinetic model, which are fitted to experimental results at methanation conditions ( T = 483 K or 493 K, p = 1.85 bar and H 2/CO = 10) over a Co/Al 2O 3 Fischer–Tropsch catalyst. A good tuning is obtained by adjusting the C–Co and H–Co binding strengths. The microkinetic modelling based on these assumptions indicates that CO is mainly converted through hydrogenation of CO and that C 2 compounds are mainly produced by insertion of CO into a metal–methyl bond. Thus, from the surface coverages and reaction rates predicted by the microkinetic modelling the mechanism can be further reduced to only include the CO insertion mechanism. Hydrogenation of CHO to CH 2O is found to be the rate determining initiation step, and insertion of CO into a metal–methyl bond is found to be the rate determining step for chain growth. By using the UBI-QEP method for calculation of activation energies, the activation barriers for dissociation of CO and hydrogenation of surface carbon are found to be too large for the carbide mechanisms to occur. However, experimental data or another theoretical method is necessary in order to support or disprove the calculated activation energies in this work.