Heat of adsorption of hydrogen on evaporated films of tungsten and of nickel

Abstract

Heats of adsorption of hydrogen on evaporated metal films have been determined at 273, 195 and 90°K with a Beeck-type calorimeter of greatly increased sensitivity, the heat capacity of which has been determined independently at each temperature. For both tungsten and nickel, the heat of adsorption falls with increasing coverage at 273"K, but is constant at 90°K. The integral heat of adsorption is independent of temperature for both metals. The present results are compared with those already in the literature.

Gas Adsorption

“…Because physical adsorption and liquefaction are related in this way, the former tends to occur only at temperatures near to the boiling point of the adsorbate under the pre vailing conditions of pressure. More precisely, if the adsorption is observed at a pressure p and p 0 is the saturation vapour pressure at the same temperature, then an appreciable fraction of a physically adsorbed monolayer is formed only when p/p 0 exceeds about 0.001 (Brennan and Graham, 1965). However, this does not necessarily apply to adsorbents possessing fine capillaries.…”

“…The second stage proceeds fairly rapidly at room temperature but does not occur at 77 °K, presumably because adsorption into this state is activated. Brennan and Graham (1966) have suggested that this adsorption consists of oxygen atoms occupying sub-surface sites on certain planes such as (100). This may be represented diagrammatically as follows: This process of place-exchange is assisted by chemisorption of an oxygen molecule from the gas phase and would otherwise be an energetically unfavourable step.…”

“…Recently, Brennan and Graham (1966) have measured heats of adsorption of oxygen at 273 and 77 °K. Their results are shown in Figure 27.…”

“…Little additional information is available for other metals in this group with the exception of nickel, which has been studied in great detail and will therefore be dealt with separately. Brennan and Graham (1966) have measured heats of adsorption of oxygen on nickel films at 77 and 273 °K. Their results are shown in Figure 31.…”

“…In the second and third columns, N(CO) is the number of carbon monoxide molecules adsorbed at 273 °K, N(Kr) is the number of krypton atoms adsorbed on the clean surface at 77 °K, and N(M) is the number of surface metal atoms in the film. The latter has been calculated from N(Kr) using the configurations proposed by Brennan and Graham (1965) and assuming the surface to consist of equal areas of the three major crystal planes. I t is evident that the majority of CO molecules occupy two surface sites on the metals studied.…”