(457f) DFT Study of the CH Hydrogenation and CH-CH Carbon Coupling Reactions on Different Surface Facets of Co Catalyst

Fischer-Tropsh is a chemical process to convert syngas (CO and H₂) into long hydrocarbon chains to use as fuel but the process suffers from large product distribution that requires expensive post processing. Here we study the reaction mechanism of hydrocarbon chain growth on Co and how different surface facets effect the reaction steps. The Fisher-Tropsh 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. Based on that analysis, the CH-CH carbon coupling and CH hydrogenation were determined to be the critical steps in chain growth, CH₄ formation, and CO utilization. In this study we use Density Functional Theory (DFT) 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 three different Co facets ((100), (110), and (211)) 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. 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.