AVS 53rd International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuA

Paper SS1-TuA4
Surface Segregation in a Polycrystalline Palladium-Copper Alloy

Tuesday, November 14, 2006, 3:00 pm, Room 2002

Session: Surface Structure and Morphology
Presenter: J.B. Miller, Carnegie Mellon University
Authors: J.B. Miller, Carnegie Mellon University
P. Ye, Carnegie Mellon University
C. Matranga, U.S. Dept. of Energy
A.J. Gellman, Carnegie Mellon University
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The surface composition of an alloy is rarely the same as its bulk. The differences are reflected in a number of important surface properties, including corrosion rates and catalytic activity. This work addresses the phenomenon of surface segregation in a polycrystalline palladium-copper alloy. Palladium-copper is of significant practical interest because of its potential as a hydrogen separation membrane for corrosive environments, like those found in coal gasification process streams. We combine X-ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering (LEIS) techniques to understand how the compositions of the surface region (SR, ~ top 3-4 atomic layers) and top layer (TL) of a bulk 70:30 (atomic) Pd:Cu alloy vary with thermal treatment and adsorption of hydrogen sulfide. On the clean surface, copper enrichment, both in the SR relative to the bulk and in the TL relative to the SR, occurs at all temperatures above 600K. Adsorption of sulfur as hydrogen sulfide results in a palladium-only TL, illustrating how the presence of an adatom can significantly affect segregation patterns. We consider our results from the perspective of thermodynamic theory, and demonstrate that consideration of the heat of mixing is important for a complete description of this system.