Fischer-Tropsh Synthesis is a chemical process that converts CO and H₂ (syngas) into long stable hydrocarbon chains to use as fuel. This process suffers from large product distribution that requires expensive post processing. In this study, the reaction mechanism of hydrocarbon chain growth on Co is investigated on different surface facets of Co in order to study their effects on the rate-limiting reaction steps. Fischer-Tropsch is a complex multi-step process so a microkinetic model for the carbide chain growth mechanism, available literature data, and the degree of rate control analysis was used to determine the rate-limiting steps for hydrocarbon chain production, CO utilization, and minimization of CH₄ production. The CH-CH carbon coupling and CH hydrogenation were determined to be the critical steps in chain growth, CH₄ formation, and CO utilization based on the degree of rate control analysis of the microkinetic model. Density Functional Theory (DFT) was used to investigate these reaction steps on cobalt catalyst to understand how different surface facets affect these reaction steps. The calculated energy landscape, reaction and activation energies for the two reaction paths are initially compared on two different Co facets ((001) and (110)) and used to determine the most effective surface structure for CH-CH carbon coupling, which leads to chain growth, and the least effective surface structure for CH hydrogenation, which leads to CH₄ formation. Other surface facets under investigation include (111) and (101). The forward activation barrier for the CH coupling reaction showed little sensitivity to cobalt surface structure (ΔE₀₀₁,[subscript F] = 1.01 eV, ΔE₀₁₁,[subscript F] = 1.08 eV) as opposed to the CH hydrogenation reaction where forward activation barriers ranged between 0.29 eV and 0.96 eV on the (001) and (101) facets respectively. This study is a part of a larger initiative to design and optimize a FTS micro-reactor for commercial use which combines atomistic DFT calculations, multiphysics modeling (mass, heat and fluid transport), experimental catalyst design, and micro-reactor design.
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