Heats of adsorption of oxygen on evaporated films of molybdenum, tungsten, cobalt and nickel at 77, 90 and 273°K, and nature of adsorbed layers

Abstract

Calorimetric measurements are reported which show that oxygen adsorbed to saturation on molybdenum and tungsten at 77°K has the same energy as oxygen adsorbed on these metals at 273°K to the same coverage. The additional coverage at 273°K, either for a surface initially saturated at 77°K or for a surface maintained at 273°K throughout the adsorption, is effectively the same in both cases and is associated with the same energy which is lower than that of the initial state and falls with increasing coverage. It is argued that the final state of the adsorbed layer on these metals at 273°K is independent of the temperature path. A model is proposed for each stage of the adsorption.

For cobalt and nickel at 77°K, despite the similarity of the saturation coverages to those obtained at 273°K, the heats of adsorption are lower than those at 273°K. This is explained in terms of the formation at 77°K of a chemisorbed layer confined to the surface proper of the metal, whereas, at 273°K incorporation of oxygen into an oxide layer occurs. Adsorption of oxygen at 273°K on a surface previously saturated at 77°K results in a further uptake amounting to 30–40 % of the initial adsorption at 77°K. It is argued that, on certain planes, the oxygen initially adsorbed at 77°K is to be found at 273°K sandwiched between the first and second layers of metal atoms, and that the metal atoms again exposed in the surface proper are able to adsorb further oxygen. Models for the adsorbed layers are proposed.