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

Abstract

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.

Catalysis for Fuels: general discussion

“…The results show that about 10% of the products would lead to the formation of water. 1 Water is thought to be activated on Cu (similar to propanol) and forms a basic surface hydroxyl that can carry out the same nucleophilic attack or H abstraction as the alkoxide on Cu, which is consistent with your statements. We reported in the manuscript (DOI: 10.1039/C6FD00226A) that the OH* groups are somewhat more basic than the alkoxide interediates and show that the barrier to activate the weakly acidic C-H bond on the bound propanal is ~14 kJ mol 1 more favorable than that of the alkoxide.…”

“…The kinetic response is consistent with a limiting step mediated by a bimolecular transition state for both condensation and esteri cation. 1 We observed no eff ects of water, suggesting that neither O* nor OH* are involved as nucleophiles. Abhishek Kumar Gupta asked: Can Cu nanoparticles be used as a catalyst for transesteri cation of vegetable oil or waste oil?…”

“…1 An example of the use of the degree of selectivity control and its derivation can be found in work on CO hydrogenation on Rh surfaces. Nora de Leeuw responded: Certainly, the microkinetic model employed here was derived entirely from computational results without any external parameters.…”

“…A similar approach has been presented in the literature. 1 Nora de Leeuw answered: We would like to analyse the degree of rate control to con rm that the adsorption/desorption is the limiting step in the production of H2. We recognise that both the adsorption of NH3 and H2 are important rates in the evolution of hydrogen, but more analysis is still required.…”

“…1 This would imply that it would quite easily liberate hydrogen gas, while maintaining the bonds to the other adsorbates at the operating temperature. This could be one of the features that makes it a good catalyst for the liberation of hydrogen gas from hydrazine.…”