Reactivity of C3Hx Adsorbates in Presence of Co-adsorbed CO and Hydrogen: Testing Fischer–Tropsch Chain Growth Mechanisms

The identity of the surface intermediates involved in chain growth during Fischer–Tropsch synthesis remains a topic of ongoing debate. In the present work we use a combination of temperature programmed reaction spectroscopy and high resolution X-ray photoemission spectroscopy to study the reactivity of C3Hx adsorbates on a Co(0001) single crystal surface in order to explore the stabilities of the different C3Hx surface intermediates and to study elementary reaction steps relevant to chain growth and chain termination. Propene (H3C–CH=CH2) and propyl (H3C–CH2–CH2–) adsorbates react below 200 K already, either by desorption of propene or by dehydrogenation to adsorbed propyne (H3C–C≡CH). Co-adsorbed H ad and CO ad do not affect the temperature at which propyl and propene react, but they do suppress the dehydrogenation pathway in favour of propene desorption. Their high reactivity under simulated FTS conditions disqualifies them as feasible intermediates for FTS, which requires long-lived intermediates to match the low monomer formation rate. Propyne, the most stable C3Hx adsorbate in the absence of CO ad, is hydrogenated to propylidyne (H3C–CH2–C≡) > 230 K when both CO ad and H ad are present. Propylidyne dimerization occurs around 313 K and produces a 3-hexyne (H5C2–C≡C–C2H5) surface intermediate which is hydrogenated to 3-hexene (H5C2–CH=CH–C2H5) above 350 K. These findings are of direct relevance to FTS: they show that the high coverage of CO ad and H ad present during the reaction influence the reactivity of CxHy adsorbates involved in chain growth and ultimately steer product selectivity. The findings provide further experimental support for the previously proposed alkylidyne chain growth mechanism on close-packed cobalt terraces: CO stabilizes CxHy growth intermediates in the alkylidyne form and growth proceeds via coupling of a long chain alkylidyne and methylidyne (CH)