Heats of adsorption on metal wires - II. The heat of adsorption of hydrogen on nickel

Abstract

The heat of adsorption of hydrogen, q , on polycrystalline f. c. c. nickel wires has been deter­mined at 86 and 279 K, and some intermediate temperatures, as a function of fractional coverage, θ . At 86 K q ( θ = 0) = 167 kJ mol -1 , decreasing linearly to 0 at θ = 0.58. This corresponds to dissociative adsorption into an immobile film with a nearest neighbour repulsive interaction energy of 38.8 kJ mol -1 . At 279 K q ( θ = 0) = 234 kJ mol -1 , and has a stepwise character with a strong decrease at θ = 0.2. These results correspond to dis­sociative adsorption into a mobile film. So the nickel surface behaves essentially as a homo­geneous set of localized sites, and is probably more homogeneous than the tungsten surface. The exposed planes are probably the lowest energy planes, (111) and (100), and the high neighbour interaction energy, compared to tungsten, may be associated with the relatively small 0.248 nm spacing. The effect of increasing mobility of adatoms is shown in the q — θ curves at the intermediate temperatures of 273, 246 and 267 K.