MODIFICATION OF THE DISSOCIATION PATHWAY OF TOLUENE ON RU(001) BY COADSORBED CO OR OXYGEN

The influence of CO on the adsorption and decomposition of toluene has been studied in the coadsorption system prepared by saturating the we ll-defined (root 3 x root 3)R30 degrees CO/Ru(001)-layer with toluene at 200 K, which then contains about 4.5 CO molecules to 1 toluene mole cule. The initial decomposition steps of toluene are drastically modif ied by the high density of coadsorbed CO in this layer. Utilizing HREE LS, TPD, Delta Phi, and LEED, and employing partially deuterated tolue ne, the first dissociation step of toluene and the formation of a C7H7 -benzyl species can be identified around 320 K, 100 K lower than in th e pure toluene/Ru(001) system, which is attributed to a destabilizing influence of CO on toluene. This eta(7)(pi+sigma) bonded (C7H7) benzyl species is the major decomposition product between 280 and 350 K. In contrast to the behavior in the pure toluene layer, two of the methyl hydrogen atoms desorb at a clearly lower temperature than the ring hyd rogens, and the abstraction of the first methyl-H which leads to the b enzyl species is clearly separated from the others, so that this speci es can be studied spectroscopically. The thermal desorption data sugge st a C7H6 species as a further intermediate, for which no direct proof can be derived from HREELS, however. Two different CO species with nu (C-O) approximate to 1985 cm(-1) and 1675-1750 cm(-1), respectively, a re observed in the coadsorbate layer; the one with the lower C-O stret ch frequency transforms into the other one by 320 K, simultaneously wi th the transition toluene-->benzyl. Some data on the thermal evolution of adsorbed toluene coadsorbed with oxygen are also reported which sh ow similar, although not quite as distinct features, albeit at lower c oadsorbate coverage ratio. This supports the assumption that the chang e of reactivity is due to an electronic effect of the electronegative coadsorbates on adsorbed toluene which may contain inductive and field -induced contributions.