Surface Carbon Hydrogenation on Precovered Fe(110) with Spectator-Coverage-Dependent Chain Initiation and Propagation

Experimentally, surface carbon hydrogenation on metallic iron forms CH₄, ethylene/ethane, and propene/propane, while previous density functional theory studies showed that these reactions are highly endothermic. This disagreement can be attributed to the shortcoming of low coverage of surface carbon atoms and the underestimated role of surface hydrogen. On a p(4 × 4) Fe(110) surface with 0.25 ML carbon coverage (4 C atoms) and other free sites filled with H atoms, we studied surface C hydrogenation with spectators and found that the formation of CH₄, ethylene/ethane, and propene/propane becomes exothermic. Coupling of CH + CH and CH₃C + CH to acetylene and propylene is favored thermodynamically. Next, CHCH and CH₃CCH can be hydrogenated into CH₂CH and CH₃CHCH, and the subsequent hydrogenation of CH₂CH and CH₃CHCH determines the formation and selectivity of alkenes and alkanes. The most important surface species is carbide HC for chain initiation or CH₃C (RC for higher homolog) for chain propagation. This model of high coverage with spectators may help understand the mechanism of heterogeneous catalysis under real conditions.