Theoretical Insights on Chain Growth Mechanisms in Fischer-Tropsch Synthesis at High CO Coverage on Ru Particles

The global demand for energy is projected to increase by over 70% by 2050 [43] . Such demands coupled with decreasing oil reserves will ultimately require sustainable strategies for the production of clean fuels. Fischer Tropsch Synthesis (FTS) is a well-established industrial process that can be used to convert syngas derived from emerging energy sources such as natural gas and biomass feedstocks into fuels and chemicals [1,2,3,4]. FTS involves the activation of CO in the presence of hydrogen to form hydrocarbon monomers that subsequently polymerize to form n-alkanes, 1-alkenes, aldehydes and alcohols. The reaction is typically carried out on group VIII metals including Fe, Co, Ru, Rh and Ni [1,2,3,5]. Despite decades of research the mechanisms involved in C-O activation and C-C bond formation are still actively debated. First-principle density functional theory (DFT) calculations were carried out herein to investigate the elementary C-C bond formation and chain propagation pathways over the model (111) terrace sites of a 119 atom Ru cluster chosen to mimic the active sites of large supported Ru clusters. The calculations were carried out at full monolayer coverages of CO to appropriately capture the high coverage conditions that are used industrially as well as in the laboratory. Both the CO insertion and carbene or “CHx insertion” mechanisms were explored. Plausible chain propagation paths via the CO insertion mechanism were explored including comparisons of direct versus hydrogen-assisted activation of CHxC-O*. The addition of hydrogen to oxygen lowered the free energy of the transition states for C-O scission in comparison with routes involving direct C-O bond activation in all cases irrespective of the saturation of the CHx* species. Simulation results indicate that the chain propagation for CO insertion proceeds via the addition of surface hydrogen (H*) to the oxygen atom of adsorbed CH3CO* in an irreversible step that controls the selectivity with an overall activation barrier ~ 140 kJ mol-1 relative to adsorbed methylidyne (CH*). Chain propagation for the CHx insertion mechanism preferentially proceeds via carbon-carbon coupling of two alkylidyne surface intermediates or by the combination of an alkylidyne with an alkylidene intermediate with low free energy barriers ~70 kJ mol-1 relative to two adsorbed CH*. While the resulting energetics reported herein indicate that CHx insertion is energetically much more favorable than CO insertion, the kinetics associated with the probability of CHx intermediates finding one another in the dense CO adlayer may limit such paths.