Fischer-Tropsch synthesis, one of the oldest and largest catalytic processes, converts syngas (CO and H2) into long-chain hydrocarbons by a combination of C-O activation and C-C coupling steps. The nature of the active sites and the reaction network remain intensely debated, hampering development of selective catalysts. In the carbide mechanism, chain-growth occurs via the coupling of CHx species and the CHx coverage needs to be sufficiently high. In the CO insertion mechanism, RCHx* groups couple with CO* before C-O scission. To investigate the structure of the cobalt particles, the cobalt surface and the possible reaction mechanisms, reaction free energies were computed using VdW-DF density functional theory. To accurately capture the reaction environments, realistic surface CO coverages are incorporated. Using the DFT-calculated adsorption and reaction energies, a dual-site microkinetic model was constructed. Over 1000 elementary reactions on both terrace and edge sites (Figure 1a) were included. Simulations at a low CO pressure predict a high CH4 selectivity. When the pressure is increased, resulting in high CO coverage, chain-growth and a CH4 selectivity in line with experiments is observed (Figure 1b). OH-assisted CO activation occurs preferably at edge sites, followed by CH* diffusion to the terraces, where chain-growth takes place. Both the carbide and the CO-insertion mechanisms contribute to chain growth. The activity increases with the number of edge sites, but only up to a limit (Figure 1c). Above 10% B5 sites, the CH*/CO* coverage ratio on the terraces approaches the thermodynamic limit.

Mechanism and active sites for cobalt-catalyzed Fischer-Tropsch synthesis

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