The Fischer-Tropsch synthesis (FTS) reaction converts synthesis gas derived from coal, natural gas, or biomass into chemicals and hydrocarbon fuels. Atomic-level insight into the mechanism of FTS can potentially lead to a predictive understanding of changes in catalyst behaviour as a function of operating conditions. Density functional theory is well suited to investigating surface reaction mechanisms, but its application to industrial FTS remains challenging, due in part to the complexity of the catalysts employed. In Fe-FTS, iron carbides form under FTS conditions, of which ?-Fe5C2 has been associated with FTS activity. In this work, we present a systematic investigation of early steps in Fe-FTS as a function of ?-Fe5C2 surface structure[1,2], incorporating carbon vacancy and step sites to investigate direct-and hydrogen-assisted dissociation of adsorbed CO, as well as subsequent C and O removal.Although the strength of CO adsorption was found to be largely insensitive to the nature of the adsorption site, significant differences in the degree of activation of the C�O bond were found, with adsorption at step sites facilitating a higher degree of coordination of CO to the surface, with a correspondingly larger increase in the C�O bond length and red-shift in the C�O stretching frequency relative to adsorption in terrace sites. The more activated CO adsorbed at step vacancy sites is associated with a significantly lower activation energy for direct dissociation. In the terrace environment, direct and hydrogen-assisted paths have comparable overall activation energies, and can be kinetically competitive depending on the surface concentration of hydrogen and empty sites. In contrast, for step sites the hydrogen-assisted route is less favourable than direct dissociation. Overall, C and O removal are strongly endothermic in the calculated coverage regime, suggesting that coverage effects will play a decisive role in determining surface reaction energetics under FTS conditions.1. P. J. Steynberg, J. A. van den Berg, and W. Janse van Rensburg, J.Phys.: Condens. Matter 20  (2008) 064238.2. M. A. Petersen, J. A. van den Berg, and W. Janse van Rensburg, J. Phys. Chem. C 114  (2010) 7863.

Influence of defect sites on elementary surface reactions in Fe-based FTS: Insights from DFT

248th National Meeting (2014)

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