Major and Minor Reactions in Fischer–Tropsch Synthesis on Cobalt Catalysts

Abstract

Minor reactions, accompanying the major reactions for building straight-chains of aliphatic hydrocarbons from the reactants CO and H₂ on the surface of cobalt catalysts, can contribute substantially to the understanding of the regime of Fischer–Tropsch synthesis. This goal affords precise mass balances, precise determination of product composition and consistent kinetic schemes for obtaining the right kinetic coefficients. The concept of self-organization of the Fischer–Tropsch regime is established from time dependence of activity, selectivity and catalyst structure. A process of thermodynamically controlled restructuring/segregation of the cobalt surface is addressed and understood as activating the catalyst and specifically, disproportionating on-plane sites into sites of lower coordination (on-top sites) and higher coordination (in-hole sites). These different sites appear to collaborate in the Fischer–Tropsch regime, with steps of coordination chemistry (comparable to those of transition metal complexes) on on-top sites and dissociation (specifically of CO) on in-hole sites and further in principle suppressed reactions on on-plane sites. This concept is developed and illustrated here with the results of several investigations such as tracing of activity and selectivity during the initial episodes of synthesis, experiments with added (¹⁴C-labeled) olefins and variation of synthesis parameters to see their specific influences. As minor reactions of coordination chemistry on on-top sites, reversible CH₂ cleavage from alkyl chains, CO insertion and ethene insertion are visualized. On on-plane sites CO methanation, olefin hydrogenation and olefin double bond shift are noticed, but much inhibited.

As compared to Fischer–Tropsch on iron catalysts, the common Fischer–Tropsch principle appears to be the inhibition of chain desorption to allow for growth reactions of the adsorbed chains. Minor reactions and detailed kinetics on iron and cobalt catalysts differ basically.