Oxygen-covered nickel films

Fig. 2. Typical curves of the relative changes of the electrical resistance of nickel films as a function of time (a) adsorption of one dose of hydrogen on the surface, partially covered by preadsorbed oxygen (b) adsorption of one dose of oxygen on the surface, covered by preadsorbed hydrogen (both at 300°K).

Nickel films completely covered with oxygen will not adsorb hydrogen when exposed to this gas immediately after oxidation, but will regain their ability to adsorb hydrogen after several hours. Hydrogen thus adsorbed is not able to hydrogenate ethylene. Heats of adsorption measurements of this type of hydrogen adsorption have not been made. 

Iron films behave in every way similar to nickel films except that upon admitting oxygen, about seven oxygen atoms are sorbed instantaneously for every crystallographic site, forming an iron oxide film seven atom layers deep, and except that the heat of adsorption of hydrogen on such an oxide covered film is almost identical with that on the clean iron surface even immediately after the oxidation has taken place. 

The complex nature of these reactions may, in part, reside in the tendency for certain metals to give surface oxide layers several molecules thick which may, or may not, be catalysts for the reaction. Thus, while silver or gold is poisoned by a thick oxide layer (115), this does not seem to be the case for nickel (112), which, in the presence of a hydrogen-oxygen mixture, is reported to be covered by an oxide film 20 A. thick. Bulk nickel oxide is known to be a catal)"st (116). 

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