The coadsorption of CO and benzene on Ru(001) at 4:1 ratio: structure and mutual influences

The coadsorption system benzene + CO on Ru(001) at a CO: benzene mixture of 4:1 has been investigated with HREELS, LEED, Δφ and TDS. At this stoichiometry an ordered ( 13 × 13)R13.9° superstructure containing one benzene and four CO molecules per unit cell is formed at 250 K. The mutual distortion of both coadsorbates is only weak as deduced from frequency shifts in the vibrational spectra. The work function change of the coadsorption layer is considerably smaller than the sum of those of the individual adsorbates at the corresponding coverage on the clean crystal. Thermal desorption shows that there is a net attraction between the two molecules in the mixed layer, since the molecular desorption temperatures of both CO and benzene are higher than from the pure, equally crowded layers. The desorption spectra of D₂ resulting from dissociation of C₆D₆ show a strong enhancement of the peak at 494 K compared to the pure benzene layer which is explained by the sudden availability of sites vacated by desorbing CO, whereas the other desorption features remain essentially unchanged. As in the pure benzene layer a kinetic isotope effect is observed in the dehydrogenation of benzene in the coadsorbate layer.