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Experiments that provide insight into the elementary reaction steps of C x H y adsorbates are of crucial importance to better understand the chemistry of chain growth in Fischer−Tropsch synthesis (FTS). In the present study we use a combination of experimental and theoretical tools to explore the reactivity of C 2 H x and C 3 H x adsorbates derived from ethene and propene on the close-packed surface of cobalt. Adsorption studies show that both alkenes adsorb with a high sticking coefficient. Surface hydrogen does not affect the sticking coefficient but reduces the adsorption capacity of both ethene and propene by 50% and suppresses decomposition. On the other hand, even subsaturation quantities of CO ad strongly suppress alkene adsorption. Partial alkene dehydrogenation occurs at low surface temperature and predominantly yields acetylene and propyne. Ethylidyne and propylidyne can be formed as well, but only when the adsorbate coverage is high. Translated to FTS, the stable, hydrogenlean adsorbates such as alkynes and alkylidynes will have long residence times on the surface and are therefore feasible intermediates for chain growth. The comparatively lower desorption barrier for propene relative to ethene can to a large extent be attributed to the higher stability of the molecule in the gas phase, where hyperconjugation of the double bond with σ bonds in the adjacent methyl group provides additional stability to propene. The higher desorption barrier for ethene can potentially contribute to the anomalously low C 2 H x production rate that is typically observed in cobalt-catalyzed FTS.

Atomic carbon on Co(0001), deposited by ethylene decomposition, forms islands with a (√3 × √3)R30°structure at low C coverage (∼0.2 ML), whereas a high C coverage (0.5 ML, saturation) induces a reconstruction of the cobalt surface. Atomic carbon weakens the adsorption of CO and H 2 , but even a saturated atomic carbon layer does not block the surface for adsorption. Carbon−carbon coupling, i.e., polymeric carbon formation, was not observed for temperatures ≤630 K on the close-packed cobalt surface. Polymeric carbon, in the form of small graphene islands, forms on the close-packed terraces after heating of an acetylene-saturated surface. Graphene also forms upon heating of an atomic carbon covered surface on which ethylene was dosed at low temperature. In this case, step edges act as a nucleation point of the graphene islands, while their growth proceeds via the addition of C 2 H x species. In both cases, hydrogenated forms of carbon rather than atomic carbon are key precursors for graphene growth. Graphene covers the cobalt surface, thereby inhibiting adsorption of CO and hydrogen completely. The described graphene formation mechanism is seen as a relevant, low temperature route to detrimental carbon that would deactivate a cobalt FT catalyst. Atomic carbon is more reactive than graphene, as it is oxidized at lower temperatures than graphene. The graphene islands formed at relatively low temperatures are of poor structural quality and contain (islands of) encapsulated cobalt atoms.

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Recently there has been a renewed interest in Co-catalyzed Fischer−Tropsch synthesis (FTS) from natural gas, coal, and biomass, because it offers a realistic alternative to crude oil as a source of transportation fuels. Efforts to understand the FT mechanism on the atomic level have mainly focused on theoretical methods, whereas experimental surface science results have only had little impact on the understanding of the mechanism. An essential step in any FT mechanism is scission of the C−O bond. On a flat Co(0001) surface direct dissociation of the CO molecule is practically impossible at FTS conditions. We have found for the first time experimentally that the C−O bond can be broken at 350 K even on the relatively inert Co(0001) surface if a C x H y group and a hydrogen atom are attached to the C-end of the C−O moiety.