Influence of Steps on the Adsorption of Methane on Platinum Surfaces

Abstract

The activated adsorption of methane was investigated on two stepped Pt(355) and Pt(322) surfaces by time-dependent in situ high-resolution X-ray photoelectron spectroscopy (XPS) combined with a supersonic molecular beam and was compared to corresponding results on Pt(111). Both stepped surfaces have five atom row wide (111) terraces but a different step orientation, namely, (111) and (100), respectively. Independent of the kinetic energy (0.45−0.83 eV) of impinging methane (CH4 or CD4), methyl is formed on all surfaces upon adsorption at 120 K. For the stepped surfaces, two different methyl species are identified from the XP spectra, which are attributed to adsorption at terrace sites and at step sites. The total initial sticking coefficients (for terrace + step sites) and the total coverages are very similar for all three surfaces. At low coverages, diffusion of methyl from the terraces to the steps is significantly stronger on Pt(355) than on Pt(322), and the step saturation coverage is higher on Pt(355). The thermal evolution of methyl was also investigated by in situ temperature-programmed XPS. Overall, an enhanced reactivity toward dehydrogenation to methylidyne is found for both terrace and step sites on the stepped surfaces, with the (111) steps of Pt(355) exhibiting the highest activity.