Reflections on the Fischer-Tropsch synthesis: Mechanistic issues from a surface science perspective

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The current paper presents a mechanistic view on important steps in the Fischer-Tropsch synthesis on cobalt catalysts, inspired by surface science studies. By revisiting the relation between activity and selectivity that results from the ASF assumption we highlight that knowledge about the number of growing chains as well as their residence time (∼growth rate) is of crucial importance to sketch a physically realistic scenario for FTS. This motivates further investigations into the microscopic scenario for FTS chain growth on fcc cobalt nanoparticles, by looking into the reaction mechanism in relation to surface structure and by determining the activation energies for key elementary steps. Such studies indicate that the modest activity of Co FTS catalysts might very well be attributable to the difficulty to remove chemisorbed oxygen from the metallic surface, rather than to dissociation of CO, which was found to proceed readily at step edge sites. Chain growth is envisaged to take place on the close-packed surfaces, with chain initiation via CH+CH to form acetylene, followed by hydrogenation to form ethylidyne, CCH3, a reaction that is shown to be promoted by co-adsorbed CO. Ethylidyne then couples with CH to form propyne, HCCCH3, etc. We propose that a fairly large number of surface sites is involved in the growth of a single chain. In such a “growth ensemble” multiple active step sites produce CHx monomer species that spill over onto the same close-packed coupling terrace, where one or only a few chains grow at the same time. In such a scenario diffusion of hydrocarbonaceous surface species is an essential step in the overall reaction sequence. We explore which factors need to be taken into account when considering of CxHy species under realistic reaction conditions. In addition, we note that the coupling reaction itself, via CH+CCnH2n+1, is a source of growing chain mobility.