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 well-defined (√3 × √3)R30° CO Ru(001)-layer with toluene at 200 K, which then contains about 4.5 CO molecules to 1 toluene molecule. The initial decomposition steps of toluene are drastically modified by the high density of coadsorbed CO in this layer. Utilizing HREELS, TPD, ΔΦ, and LEED, and employing partially deuterated toluene, 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 the pure toluene Ru(001) system, which is attributed to a destabilizing influence of CO on toluene. This η₇(π + σ) 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 hydrogens, and the abstraction of the first methyl-H which leads to the benzyl species is clearly separated from the others, so that this species can be studied spectroscopically. The thermal desorption data suggest a C₇H₆ species as a further intermediate, for which no direct proof can be derived from HREELS, however. Two different CO species with v_CO ≈ 1985 cm^-1 and 1675-1750 cm^-1, respectively, are observed in the coadsorbate layer; the one with the lower CO stretch frequency transforms into the other one by 320 K, simultaneously with the transition toluene → benzyl. Some data on the thermal evolution of adsorbed toluene coadsorbed with oxygen are also reported which show similar, although not quite as distinct features, albeit at lower coadsorbate coverage ratio. This supports the assumption that the change of reactivity is due to an electronic effect of the electronegative coadsorbates on adsorbed toluene which may contain inductive and field-induced contributions.