Identification of adsorbed phenyl (C₆H₅) groups on metal surfaces: Electron-induced dissociation of benzene on Au(111)

We have investigated thermal and electron-induced chemistry of benzene (C₆H₆) adsorbed on a Au(111) surface. Thermal desorption of benzene occurs in three desorption peaks: monolayer at 239 K, bilayer at 155 K, and multilayer films at 151 K. Electron-induced dissociation (EID) has been reported previously to selectively break a single C-H bond in molecules present in physisorbed layers and condensed films on metal surfaces, and we investigate whether EID at an incident energy of 30 eV can cleanly prepare adsorbed phenyl (C₆H₅(a)) groups on the surface at low temperatures (∼90 K). We use infrared reflection-absorption spectroscopy (IRAS) to show unequivocally that adsorbed phenyl groups can be formed by this procedure. Phenyl groups on Au(111) are bound with the molecular (ring) plane perpendicular to the Au surface plane, with the molecular z-axis tilted away from the surface normal. In contrast to previous reports of the chemistry of phenyl groups adsorbed on Cu(111) and Ag(111) surfaces, we find that adsorbed phenyl groups are stable only until 165 K on Au(111). At higher temperatures, phenyl groups undergo coupling reactions to form adsorbed biphenyl (C₆H₅-C₆H₅) species which desorb intact from the surface at 400 K. While C-H activation (bond cleavage) on Au surfaces is difficult, hydrogenation and C-C coupling reactions are facile. Diffusion of aryl and alkyl intermediates to Au sites could result in immediate coupling and possibly desorption of products. This implies that Au atoms may play a more important role in bimetallic hydrocarbon conversion catalysis than simply blocking reactive sites.