A Mechanistic Study of the Fischer–Tropsch Synthesis Using Transient Isotopic Tracing. Part-1: Model Identification and Discrimination

Abstract

A steady state isotopic transient kinetic analysis (SSITKA) of the Fischer–Tropsch synthesis over Co/Ru/TiO2 catalyst is reported by using 13CO and D2. Besides a qualitative interpretation of the transients, model identification and discrimination are mainly based on numerical modeling. From this, it is concluded that two single-C species are present on the catalyst surface, Cα,ads and Cβ,ads. These species both participate in the formation of methane and of higher hydrocarbons. The heterogeneity of the catalyst surface is limited to these two single-C species. Only one type of chain-growth site is present in a low concentration compared to the surface concentrations of COads, Cα,ads, and Cβ,ads. The H-content of Cα,ads and Cβ,ads is assessed by comparing the simulated transient for the incorporation of the D-labeling into methane with experiments. In case the stepwise hydrogenations of Cads to CH4 are irreversible, Cα,ads and Cβ,ads are H-free species. In case these reactions are reversible, the H-exchange between Cads, CHads, CH2,ads, and CH3,ads is fast compared to the net formation of methane, and the H-content of Cα,ads and Cβ,ads cannot be assessed. The most probable mechanism for the Fischer–Tropsch reaction resulting from this study is used in a next paper (Part 2) for the quantification of the kinetic parameters. In that paper, a systematic analysis of these parameters yields detailed mechanistic insight into the Fischer–Tropsch synthesis reaction.