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 (C6H 6) adsorbed on a Au(Ill) surface. Thermal desorption of benzene occurs in t hree desorption peaks: monolayer at 239 K, bilayer at 155 K, and multilayer films at 151 K. Electron-induced dissociation (EID) has been reported prev iously to selectively break a single C-H bond in molecules present in physi sorbed layers and condensed films on metal surfaces, and we investigate whe ther EID at an incident energy of 30 eV can cleanly prepare adsorbed phenyl (C6H5(a)) groups on the surface at low temperatures (similar to 90 K). We use infrared reflection-absorption spectroscopy (IRAS) to show unequivocall y that adsorbed phenyl groups can be formed by this procedure. Phenyl group s 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 n ormal. In contrast to previous reports of the chemistry of phenyl groups ad sorbed 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 gro ups undergo coupling reactions to form adsorbed biphenyl (C6H5-C6H5) specie s 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 react ions are facile. Diffusion of aryl and-alkyl intermediates to Au sites coul d result in immediate coupling and possibly desorption of products. This im plies that Au atoms may play a more important role in bimetallic hydrocarbo n conversion catalysis than simply blocking reactive sites.