Adsorption of hydrogen on platinum

Abstract

The adsorption of H 2 and D 2 on Pt field emitters has been studied from 4.2 to 320 K. Adsorption of a full layer at low temperature leads to a work function decrease of Δφ = − 0.3 eV relative to clean Pt. On heating φ increases gradually and reaches a maximum Δφ = 0.15 eV at 250 K. Desorption is complete at 320 K. There is no adsorption hysteresis. The electronegative entity has a heat of adsorption of 16 kcal/mole, the electropositive species shows a spread in adsorption energies ranging from very low values corresponding to physisorption to a maximum of 15 kcal/mole. These results confirm Mignolet's findings of a moderately bound electronegative component at low and a weakly bound electropositive component at high coverages. Arguments are presented to identify the electronegative component with chemisorbed H atoms and the electropositive component with H 2 held by charge transfer forces. Diffusion for the electropositive component sets in at > 20 K; diffusion for the electronegative species commences at 100 K, corresponding to an activation energy of roughly 4.5 kcal, or to a ratio of solE diff E ads = 0.08 , if the diffusing species is H.