Harald Kirsch, Xunhua Zhao, Zefeng Ren, Sergey V. Levchenko, and R. Kramer Campen:
J. Phys. Chem. C 120 (43), 24724–24733 (2016), pp.10;
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,. 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.
The original publication is available by link DOI: 10.1021/acs.jpcc.6b07088