Adsorption and Decomposition of Ethene and Propene on Co(0001): The Surface Chemistry of Fischer-Tropsch Chain Growth Intermediates

Experiments that provide insight into the elementary reaction steps of CxHy 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 C2Hx and C3Hx 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 COad 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, hydrogen lean 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 sigma-bonds in the adjacent methyl group provides additional stability to propene. The higher desorption barrier for ethene can potentially contribute to the anomalously low C2Hx production rate that is typically observed in cobalt-catalyzed FTS.