Estimates of the Rate Coefficients for Chain Initiation, Propagation, and Termination during Fischer-Tropsch Synthesis over Ru/TiO2

Abstract

Transient response isotopic tracer experiments have been used to study chain growth during Fischer-Tropsch synthesis over an Ru/TiO2 catalyst. This involves observation of the incorporation of 13C into reaction products after an abrupt switch from 12CO/D2 to 13CO/D2 in the feed. Values for the rate coefficients for initiation, propagation, and termination are determined by fitting theoretically generated model curves to the observed transient responses. The rate coefficient for chain initiation is independent of temperature and D2/CO ratio. The rate coefficients for propagation and termination increase with temperature. The rate constant for propagation is not affected by the D2/CO ratio. The rate coefficient for termination increases linearly with increasing D2/CO ratio. The activation energy for chain propagation is 8 kcal/mol, whereas the activation energy for chain termination is 20 kcal/mol. The relative values of these two activation energies explains the observed decrease in chain growth probability, α, with increasing temperature. Coverages by reaction intermediates are also estimated. The dominant species are monomeric building units which occupy 0.3-0.6 ML. Alkyl species that are the direct hydrocarbon product precursors occupy < 0.2 ML and adsorbed CO covers 0.7 ML.