Key Role of CO Coverage for Chain Growth in Co-Based Fischer–Tropsch Synthesis

Fischer–Tropsch synthesis converts CO and H2 to long-chain hydrocarbons. The reaction mechanism, a combination of C–O scission, C–C coupling, and hydrogenation steps, and the nature of the active sites remain intensely debated. In this work, we report a comprehensive, dual-site microkinetic model including more than 600 reversible reactions. Our model explicitly accounts for the high CO surface coverage under the reaction conditions by including a CO saturation coverage in the underlying DFT calculations. The model predictions match experimental kinetic observations with a methane selectivity of 18%, a chain growth probability of 0.83, a turnover frequency of 0.084 s–1, and an activation energy of 107 kJ/mol. A degree of rate control analysis identifies 12 rate-controlling steps, highlighting the challenges in building compact kinetic models based on one or two rate controlling steps. In the dominant reaction mechanism, CO is activated both at B5 step sites and at the terrace sites via H- and hydroxyl-assisted pathways. Chain growth occurs on the crowded terraces predominantly via CH coupling to alkylidine chains. While B5 step sites facilitate CO activation, a small concentration of 5% is sufficient to establish a quasi-equilibrium CH coverage on the terraces and higher concentrations do not notably change the model predictions.