The chemisorption of hydrogen on nickel

Abstract

The pressure of hydrogen in equilibrium with hydrogen adsorbed on nickel films produced by evaporation was measured by means of an ionization gauge. Observations were made at tem­peratures up to 200 °C and included a previously unexplored range of surface coverages, θ , down to about θ ═ 0⋅005. From the results the molar and differential entropies of the adsorbed hydrogen at 25 °C and the integral and isosteric heats of adsorption were calculated. The molar entropy in the range θ ═ 0 to 0⋅4, agreed satisfactorily with the view that the adsorbed hydrogen was localized on sites which were energetically homogeneous. However, the entropy and the heat of adsorption indicated that there were sites of higher adsorption potential which amounted to about 1⋅5 % of the surface. As θ increased from 0⋅4 to 1⋅0 the observed molar entropy at 25 °C indicated a progressive increase in the proportion of the adsorbed hydrogen which, at any moment, was migrating over the surface. This proportion, and the activation energy for migration, were calculated by comparison with the entropies of ideal systems. Near complete coverage the adsorbed hydrogen exhibited the behaviour of a two-dimensional gas. Indeed, over the range θ ═ 0⋅5 to 1⋅0 the strength of the hydrogen-metal bond was but little influenced by the instantaneous position of the hydrogen atom relative to the surface nickel atoms. In the range θ ═ 0 to 0⋅77 it was found that at a fixed fractional coverage the heat of adsorption was independent of temperature.