Surface potential of hydrogen adsorbed on platinum films

Abstract

The surface potential of adsorbed hydrogen on evaporated platinum films has been measured as a function of temperature and amount adsorbed. For films prepared by condensation at 78 K, three different adsorbed species are identified, an electronegative and an electropositive adatom and, at higher pressures, an electropositive molecular species. The electronegative form has a higher sticking probability, the largest¹⁰ heat of chemisorption (71 kJ mol^–1) and is confined to high-index planes; the electropositive adatoms have a lower heat of 36 kJ mol^–1 and the molecular species one of 5 kJ mol^–1. The dipole moments are in the ratio: –1.0 : + 1.3 : + 0.57. Previous, apparently conflicting, results of other workers who have identified only two adspecies can now be reconciled. On films formed by condensation at 273 K, the adsorption of the electronegative species is much less owing to the decreasing extent of high-index planes.