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Surface Science 69 (1977) 712-713 0 North-Holland Publishing Company SURFACE SEGREGATION IN ALLOYS Dilute solid solutions of vanadium in palladium Received 18 July 1977 The equilibrium surface composition of an alloy can be quite different from its bulk composition. This phenomenon, known as surface segregation, is quite impor- tant in catalysis [I] and in metallurgy [2] and has attracted considerable recent interest [3]. In addition to extensive experimental work, much effort has been directed to developing theories capable of predicting the equilibrium surface com- position of an alloy. A large reliable data base is required to test these theories. Un- fortunately, most of the published experimental results concern alloys which have been found to exhibit segregation. Few results on non-segregating alloys have been published, possibly because they seem less interesting. Yet these null results are as important as the positive results for testing of theory. In this brief note we report a negative surface segregation result: vanadium does not segregate to the surface of palladium. The three polycrystalline alloys studied were prepared by arc-melting 1 to 2% V with Pd. After melting, the alloys were spark cut and polished to a thickness of 10 , 200 400 bO0 ENERGY (eV) Fig. 1. Derivative Auger spectrum of Pd-1.7% V taken at 1100°C after annealing for 15 min. The surface is slightly contaminated with S and Si. No segregation of V to the surface is evi- dent. 712

Surface segregation in alloys: Dilute solid solutions of vanadium in palladium

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Surface Science 69 (1977) 712-713

0 North-Holland Publishing Company

SURFACE SEGREGATION IN ALLOYS

Dilute solid solutions of vanadium in palladium

Received 18 July 1977

The equilibrium surface composition of an alloy can be quite different from its bulk composition. This phenomenon, known as surface segregation, is quite impor- tant in catalysis [I] and in metallurgy [2] and has attracted considerable recent interest [3]. In addition to extensive experimental work, much effort has been directed to developing theories capable of predicting the equilibrium surface com- position of an alloy. A large reliable data base is required to test these theories. Un- fortunately, most of the published experimental results concern alloys which have been found to exhibit segregation. Few results on non-segregating alloys have been published, possibly because they seem less interesting. Yet these null results are as important as the positive results for testing of theory.

In this brief note we report a negative surface segregation result: vanadium does

not segregate to the surface of palladium. The three polycrystalline alloys studied were prepared by arc-melting 1 to 2% V

with Pd. After melting, the alloys were spark cut and polished to a thickness of 10

,

200 400 bO0

ENERGY (eV)

Fig. 1. Derivative Auger spectrum of Pd-1.7% V taken at 1100°C after annealing for 15 min. The surface is slightly contaminated with S and Si. No segregation of V to the surface is evi-

dent.

712

J.J. Burton /Surface segregation in alloys 713

mil. The procedures for determining equilibrium surface compositions, in a 3 X

lo-” Torr vacuum, were the same which we followed in our earlier work where we showed that Au segregates to the surface of Ni [4] and that Cr, Fe, and Ni do not segregate to the surface of Pt [S] .

We found that at temperatures between 700 and 1lOO’C the equilibrium surface has essentially the same composition as the bulk. A typical Auger spectrum is shown in fig. 1. Furthermore, no temperature dependence was seen in the surface composition over this entire temperature range. Therefore, we conclude that V does not segregate to the surface of Pd.

J.J. BURTON

Exxon Research and Engineering Company, Linden, New Jersey 07036, USA

References

[l] J.J. Burton and E. Hyman, J. Catalysis 37 (1975) 114. [2] M.P. Seah, Surface Sci. 53 (1975) 168. [3] S.H. Overbury, P.A. Bertrand and G.A. Somorjai, Chem. Rev. 75 (1975) 547. [4J J.J. Burton, C.R. Helms and R.S. Polizzotti, J. Vacuum Sci. Technol. 13 (1976) 204. [5] J.J. Burton and R.S. Polizzotti, Surface Sci. 66 (1977) 1.