CH₂ Stabilized at Steps on Ru(0001) by Coadsorbates

Insight into the controls of stability of one-carbon moieties adsorbed on transition-metal surfaces is important in the optimization of such industrially important processes such as the Fischer–Tropsch (FT) synthesis, that is, (2n + 1)H2 + nCO ⇄metal CnH(2n+2) + nH2O. While the broad steps of FT synthesis are clear, CO dissociatively adsorbs on steps on transition-metal surfaces, this carbon is hydrogenated, one-carbon groups are coupled, and the resulting larger molecule desorbs, a deeper description of the mechanism has proven challenging. In particular, while experiment and calculation of barriers for coupling reactions suggest that step-bound CH2 should be the chain growth one-carbon species, calculations of CH2/CH relative stability at low surface coverages suggest CH2 dissociation is too rapid to allow such a pathway and thus CH is the likely candidate. In this study, we characterize the dissociation of CH2 adsorbed at steps on the Ru(0001) surface in ultrahigh vacuum using laser-based technique vibrational sum frequency (VSF) spectroscopy and electronic structure calculation. Experimentally, we find the barrier for CH2 dissociation to be 0.47 eV, 3× larger than the calculated barrier for dissociation of an isolated CH2 on a terrace on the Ru(0001) surface. However, both our experiment and real application steps are likely saturated by adsorbed carbon species. For such a system, we show that both the barrier for CH2 dissociation and that for the diffusion of CH away from the steps each increase 2–3×. This result highlights the large influence of coadsorbates on step-bound one-carbon moieties and provides a means of reconciling previous apparently contradictory results on the FT synthesis