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

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Abstract

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.

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