Activated adsorption of methane on Pt(1 1 1) —an in situ XPS study

We have investigated the activated adsorption of methane on Pt(1 1 1) by the combination of a supersonic molecular beam and in situ high-resolution X-ray photoelectron spectroscopy at the German synchrotron radiation facility BESSY II. On exposing the surface to a methane beam with kinetic energies between 0.30 and 0.83 eV, CH₃ is formed as a stable species at 120 K; upon heating, at around 260 K the adsorbed methyl partly dehydrogenates to CH and partly recombines to methane, which desorbs. Upon adsorption at 300 K, CH is directly formed as a stable surface species. To verify the chemical identity of CH as an intermediate, we have also investigated the thermal evolution of a saturated ethylene layer. Upon heating, at ∼290 K partial ethylene desorption and the formation of ethylidyne is clearly observed in the spectra, as expected from the literature. From the binding energies and also from the vibrational signature of the C 1s spectra, an unequivocal assignment of the various surface species is possible. Measurements of the sticking coefficients of methane show that the saturation coverage at 120 K depends on the kinetic energy of the molecule; furthermore, the sticking coefficient for vibrationally excited molecules is strongly enhanced.