Key Role of CO Coverage for Chain Growth in Co-Based Fischer–Tropsch Synthesis
Konstantijn T. Rommens, G. T. Kasun Kalhara Gunasooriya, Mark Saeys
Abstract
Fischer–Tropsch synthesis converts CO and H 2 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 B 5 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 B 5 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.