The dissociative adsorption of hydrogen on defect-"free" Pt(111)

The interaction of hydrogen with an (almost) defect-free Pt(111) surface (step density ~ 0.1%) is revisited in a combined thermal energy atom scattering/thermal desorption spectroscopy (TEAS/TDS) study. We propose a novel kinetic precursor-mediated adsorption/desorption model for hydrogen/Pt(111) to reconcile seemingly conflicting results, such as extremely different dissociative adsorption kinetics at 25 and 155 K. Up to a perpendicular energy of (at least) 60 meV, highly relevant for hydrogenation reactions, the initial sticking probability scales with perpendicular energy to the power 1.9. This atypical behaviour is attributed to probing larger corrugation amplitudes at higher normal energy, leading to scattering of hydrogen into a dynamic precursor prior to dissociation and thus to increased trapping. Scrutiny of the data demonstrates that only a small minority of the surface sites (most probably steps) is active in dissociation. The observed decay of the heat of adsorption with coverage indicates strong repulsion between hydrogen atoms. The TDS-spectra of hydrogen from the defect-'free' Pt(111) consist definitively of a single (β2-)peak in contrast to the frequently measured double (β1, β2-)peak structure and at variance with the yet widely accepted conjecture that repulsive interactions lead to double (β1, β2-)peak structures in TDS-spectra. TDS-spectra simulated by applying the micro-reversibility principle and using TEAS-data are in agreement with the experimental ones. The TEAS-data, probing hydrogen whilst on the surface, are thus consistent with TDS-data, probing hydrogen after leaving the surface.