The adsorption of benzene on Ru(001)

The adsorption of benzene on Ru(001) in the temperature range 120-250 K has been studied with HREELS, TDS, LEED and ΔΦ measurements. For all coverages θ, benzene is found to be adsorbed in a configuration with the molecular plane oriented parallel to the surface. As a function of θ, benzene forms ordered overlayers ((2 3×2 3)R30° for θ<0.07; ( 7× 7)R19.1° and incommensurate both close to saturation, θ≅0.14, where θ, the ratio of benzene molecules to Ru surface atoms, is obtained from a comparison with hydrogen desorption). These coverage ranges also reflect themselves in changes of the vibrational spectra: clearly different spectra are recoreded for the two coverage ranges. Sticking appears to proceed via a precursor state. The dipole moment of adsorbed benzene molecules at low θ, as deduced from the initial slope of ΔΦ-signal versus coverage, is 3.2 debye/molecule. Thermal desorption measurements indicate that adsorbed benzene dehydrogenates in steps but also that the surface chemistry is different initial coverages and, therefore, starting states. Desorption of molecular benzene at T=360 K occurs above half monolayer coverage. Isotopic mixing experiments of C₆H₆ and C₆D₆ (HREELS and TDS) show that at saturation no difference exists pre- and post-adsorbed isotopic species. In thermal desorption, an isotope effect of desorbing H₂ and D₂ is observed for benzene dissociation above 320 K with the H species dissociating at lower T. Hydrogen transfer reactions between the molecules do not occur at 250 K but appear to start at 340 K.