Abstract
Abstract Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe3C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111), 1 1 ¯ 1 , and (010) terminations of ϴ-Fe3C. On the other hand, H-assisted CO dissociation is preferred on the 0 1 ¯ 1 , (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the ( 0 1 ¯ 1 ) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28% of the surface enclosing a Wulff nanoparticle of ϴ-Fe3C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature ϴ-Fe3C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than α-Fe (known to be unstable under Fischer-Tropsch conditions) and the χ-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.
Highlights
The growing trend to diversify the feedstock mix to cover the energy and transportation fuel demand has led to large-scale chemical processes based on the Fischer-Tropsch (FT) synthesis reaction [1]
This is a direct consequence of the interstitial C atoms present at or below the surface
The surfaces that exhibit a step-edge like geometry (B5-like site) favor direct CO dissociation
Summary
The growing trend to diversify the feedstock mix to cover the energy and transportation fuel demand has led to large-scale chemical processes based on the Fischer-Tropsch (FT) synthesis reaction [1]. In FT synthesis, synthesis gas (a mixture of carbon monoxide and hydrogen) obtained from natural gas and coal is converted to transportation fuels and base chemicals [2]. Fe is attractive because of its cost advantage in comparison to Co and Ru, and for its high activity in the water-gas-shift (WGS) reaction. The latter aspect is an advantage when synthesis gas compositions with low H2/CO ratios such as derived from coal gasification are to be converted. It is crucial to understand the nature of the different iron carbide phases and the mechanism by which they convert carbon monoxide with hydrogen to hydrocarbons
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