Surface potential study of the chemisorption of hydrogen on nickel films

Abstract

The static capacitor method has been used to record the rapid transient surface potential changes that occur when hydrogen interacts with the surface of an evaporated nickel film. At temperatures of 300°K and below, there is no evidence of incorporation of hydrogen by the metal as was previously observed with oxygen as adsorbate. At 90°K, hydrogen is chemisorbed dissociatively; the adatoms (the β species) have a negative surface potential and are strongly bound to the metal (adsorption heat of ca. 30 kcal/mole decreasing with coverage). At lower coverages, there is preferential saturation by the β species of one group of adsorption sites, probably at a specific crystal plane; with unsintered films later doses of gas are adsorbed partly on the remaining unsaturated planes as β adatoms and partly as molecules (the αγ species) with a positive surface potential, on top of the primary β-layer on this specific plane. Sintering a film forms a plane which adsorbs the αγ species but is inactive for dissociative adsorption of β-atoms. At higher pressures, molecular αγ-adsorption also occurs on the β layer present on the other planes. The positive increment observed transiently on addition of a dose of gas is a measure of the amount of αγ-species adsorbed and the decay of surface potential is associated with the migration of this species from the outer surface of the film to the unsaturated internal surfaces.

The surface diffusion coefficient of the αγ-species is 1.5 × 10⁴Å sec^–1 at 90°K and the activation energy for mobility is 3.4 kcal/mole. Increase of pressure in the gas phase effects increases in the amounts adsorbed of the αγ species at 90°K and the work function is thereby reduced to that of the clean metal for some films; on increase of temperature the αγ species are desorbed and the normal negative s.p. (max) value is recovered. The main factor causing the concavity of the s.p. isotherm is the distribution of adsorbate between the outer and inner surface, and not the depolarization effects of parallelly-aligned surface dipoles.