Monomer Formation Model versus Chain Growth Model of the Fischer–Tropsch Reaction

One of the great challenges in molecular heterogeneous catalysis is to model selectivity of a heterogeneous catalytic reaction based on first principles. Molecular kinetics simulations of the Fischer–Tropsch reaction, which converts synthesis gas into linear hydrocarbons, demonstrate the need for microkinetics approaches that do not make a priori choices of rate controlling steps. A key question pertaining to this reaction, in which hydrocarbons are formed through consecutive insertion of adsorbed CH_x monomers into adsorbed growing hydrocarbon chains, is whether the CO consumption rate depends on the rate of the CH_x insertion polymerization process. Microkinetic theory of this heterogeneous catalytic reaction based on quantum-chemical data is used to deduce expressions for the CO consumption rate and chain growth parameter α in the two limiting cases where chain growth rate is fast compared to the formation of CH_x (monomer formation limit) or where the reverse relation holds (chain growth limit). The conventional assumptions that CH_x formation is rate controlling and that change in CO coverage due to reaction is negligible lead to substantial overestimation of the rate of CO consumption. It appears that intermediate reactivity of the catalytic reaction center, with neither too low nor too high activation energies for C–O bond cleavage, and low reagent gas pressure lead to such monomer formation limiting type behavior, whereas maximum rate of CO consumption is found when chain growth rate is limiting.