Oxygen and Oxide Surfaces

Most of the kinetic results so far obtained upon oxygen exchange are summarized in Figs. 3 to 6, which also show the kinetics of the equilibration reaction plotted to the same scale. Before attempting an interpretation of these observations, it is desirable to consider first the nature and extent of the oxide surface and the adsorption of oxygen thereon and also to review briefly other experimental evidence regarding the interaction between gas and surface. 

It was, in most cases, found that the rate of oxygen adsorption was too fast for measurement at the temperatures of the exchange reactions, and most work was therefore done at temperatures around 120-250°. The saturation coverages at temperatures and pressures typical of the exchange experiments are given in Table I and it is seen that in all cases the coverage is low in terms of the B.E.T. monolayer capacity. 

In the cases of Cr203, NiO, and Fe2Ch it was possible (24) to follow the kinetics of the adsorption of 02 at constant pressures around 6 cm. It was found that the uptake q followed the Elovich equation  

A Exchange on Cr203 sample A, outgassed at various temperatures between 385 and 520° Po, = 6.5 cm. 

Using this equation, ln(r.) was evaluated at several temperatures at known coverages 6 = q/qx where qx is the equilibrium coverage at P0i = 6 cm.). The activation energy for adsorption, Ea, was evaluated from the ln(r,) vs. l/T plots, and it was found that at constant pressure the activation energy increased in a roughly linear fashion with coverage 

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