Surface Composition of Equilibrated Alloys

Ever since it was demonstrated, both experimentally and theoretically, that in equilibrated Ni-Cu films the surface composition can differ substantially from that of the bulk (4a 4d)—and this was also found for alloy powders (5)—it has been generally accepted that a catalytic study of alloys should be accompanied by a study of the surface concentration of the alloys. 

Equilibrium in the surface layer will in practice often be reached, because it has been shown (6) that the surface of thin metal films can be equilibrated by heating for about 30 min at a temperature of 0.3Tm, where Tm is the melting point (K). 

Of course, researchers had wondered about the surface composition before. Takeuchi et al. (7a, 7b) were the first to provide experimental evidence of the surface composition being different from the bulk composition in Ni-Cu catalysts by reaction with HC1. In a later paper (7c) they concluded that the 

A difference in composition between surface and bulk had been suggested by Tuul and Farnsworth (S) in order to explain the decreases in activity of Ni-Cu alloys upon heat treatments at high temperatures (500°C). However, the possibility cannot be ruled out that these results were due to impurities in the surface (9). 

Recent developments in the use of Auger electron spectroscopy (AES) have improved the possibilities for a quantitative study of the surface composition (lOa-IOd). By employing an internal standard (77) many of the ambiguities of previous applications have been eliminated. The application of AES has greatly improved the reliability of surface composition determinations compared with previous results. 

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