First-Principles Investigation of C–H Bond Scission and Formation Reactions in Ethane, Ethene, and Ethyne Adsorbed on Ru(0001)

We have studied all possible elementary
reactions (including isomerization reactions) involved in the interaction
of CH4 (methane), CH3CH3 (ethane),
CH2CH2 (ethene), and CHCH (ethyne) with the
Ru(0001) surface using density functional theory based first-principles
calculations. Site preference and adsorption energies for all the
reaction intermediates and activation energies for the elementary
reactions are calculated. From the calculated adsorption and activation
energies, we find that dehydrogenation of the adsorbates is thermodynamically
favored in agreement with experiments. Dehydrogenation of CH (methylidyne)
is the most difficult in the dehydrogenation of CH4 (methane).
CH3CH3 (ethane), CH2CH2 (ethene), and CHCH (ethyne) dehydrogenate through the CH3C (ethylidyne) intermediate. Of the five possible pathways for the
production of CH3C (ethylidyne), the CH2CH (ethenyl)–CH2C (ethenylidene) pathway is the most dominant. In the case
of ethene, the ethynyl–ethenylidene pathway is also the dominant
pathway on Pt(111). Comparison of α and β-C–H bond
scission reactions, important for the Fischer–Tropsch process,
shows that alkenes should be the major products compared to the formation
of alkynes. Dehydrogenation becomes slightly favorable at lower coverages
of the hydrocarbon fragments while hydrogenation becomes slightly
unfavorable. In addition to resolving the dominant pathways during
decomposition of the above hydrocarbons, the activation energies calculated
in this paper can also be used in the modeling of processes that involve
the considered elementary reactions at longer length and time scales.