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

Abstract

The interaction of hydrogen with an (almost) defect-free Pt(111) surface (stepdensity ∼ 0.1%) is revisited in a combined thermal energy atom scattering/thermal desorptionspectroscopy (TEAS/TDS) study. We propose a novel kinetic precursor-mediatedadsorption/desorption model for hydrogen/Pt(111) to reconcile seemingly conflictingresults, such as extremely different dissociative adsorption kinetics at 25 and 155 K. Up to aperpendicular energy of (at least) 60 meV, highly relevant for hydrogenation reactions, theinitial sticking probability scales with perpendicular energy to the power 1.9. This atypicalbehaviour is attributed to probing larger corrugation amplitudes at higher normal energy,leading to scattering of hydrogen into a dynamic precursor prior to dissociationand thus to increased trapping. Scrutiny of the data demonstrates that only asmall 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 hydrogenatoms. 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-)peakstructure and at variance with the yet widely accepted conjecture that repulsive interactions lead todouble (β1, β2-)peak structures in TDS-spectra. TDS-spectra simulated by applying the micro-reversibilityprinciple and using TEAS-data are in agreement with the experimental ones. TheTEAS-data, probing hydrogen whilst on the surface, are thus consistent with TDS-data,probing hydrogen after leaving the surface.