The Uptake of Oxygen by Metals and Metallic Oxides

The uptake of oxygen by metals at temperatures above 500° usually obeys a parabolic law, but the trend at lower temperatures is towards an initial rapid reaction followed by a very slow uptake. At room temperature the reaction usually amounts to a few layers only. The kinetics most commonly observed in the region of room temperature are of the Roginsky-Zeldovich (SS) type 

Uranium dioxide is another case where more than a monolayer of oxygen is taken up at room temperature, as shown by Anderson, Roberts, and Harper 4 )- This oxide, which has a fluorite lattice, is known to exist at rather higher temperatures as a nonstoichiometric oxygen-excess oxide with the extra oxygen ions in interstitial sites 4S). As with cuprous oxide, one may imply that chemisorption at 20° is to be looked upon as incipient formation of the appropriate stable nonstoichiometric state, so that, for UO2, it is presumed that a postmonolayer uptake is made possible by virtue of the oxygen first adsorbed having subsequently entered interstitial positions 42). The same logarithmic law [Eqs. (6) and (7)] was observed. 

The distinctions most easily drawn are those between physically and chemically adsorbed oxygen. Beebe and Dowden (44) showed that this could be achieved calorimetrically. With chromia at —183°, a slow liberation of heat without any further uptake of oxygen gave evidence of the transformation from the physically adsorbed to a chemisorbed state. The heat of physical adsorption was 4 kcal./mole, but for the chemisorbed state they derived 25 kcal./mole. A very similar transformation of physically adsorbed oxygen (3-4 kcal./mole) to chemisorbed oxygen (20-55 kcal./mole) has been observed calorimetrically with uranium dioxide at — 183° by Ferguson and McConnell (4 ). 

A clear case of different forms of chemisorbed oxygen is provided by recent studies of zinc oxide (48, 49). The quantities of oxygen adsorbed by zinc oxide are very small, much less than 1% coverage, but the uptake can be conveniently studied at low pressures using a Pirani gauge. The adsorp- 

Heaifi of Adsorption of Ethylene at 30° on Cobalt Oxide Carrying Presorbed Oxygen 

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