THE ADSORPTION AND REACTION OF TOLUENE ON RU(001), AND COADSORPTION WITH CO

Layers of chemisorbed and physisorbed toluene on Ru(001) and their thermal evolution have been studied in the temperature range 120-410 K with vibrational spectroscopy, LEED, DELTA-phi-measurements, and thermal desorption/reaction spectroscopy. We find that chemisorbed toluene is oriented parallel to the surface, with the surface bond being dominated by the pi-electrons of the aromatic ring. The first physisorbed layer is also parallel to the surface while higher layers have a more random orientation. Adsorption of toluene is accompanied by a strong decrease of the Ru work function; a value of DELTA-phi = -1.60 eV is obtained for the saturated chemisorption layer, and the surface dipole per admolecule is very similar to that of benzene. Most frequency shifts of the vibrational modes relative to the gas phase values are small. At saturation coverage a poorly ordered (3 x 3) LEED pattern is observed between 250 and 330 K. Thermal dissociation appears to lead to dehydrogenation of the methyl group first, which is followed by stepwise dehydrogenation of the aromatic ring as for benzene. All 3 hydrogen atoms of the methyl group desorb within one broad peak, but there is evidence from HREELS and DELTA-phi to support the assumption that one of these hydrogen atoms dissociates before the main desorption peak. Exchange of hydrogen atoms starts below all dissociation processes. No molecular desorption of toluene is observed for the pure chemisorbed layer at any coverage. The (3 x 3) order is considerably enhanced if CO is coadsorbed at 300 K. In such a composite layer two different CO species are observed with C-O stretching frequencies at 1850 and 1600 cm-1, respectively. The low-frequency CO species has never been observed before for coadsorption with hydrocarbons on this surface. Its presence can be directly correlated with the formation of the (3 x 3) ordered overlayer; it disappears at the onset of decomposition of toluene at approximately 330 K. A model for the coadsorption structure is proposed.