Surface segregation in a polycrystalline Pd70Cu30 alloy hydrogen purification membrane

X-ray photoelectron spectroscopy (XPS) and low energy ion scattering spectroscopy (LEISS) have been used to study the effects of various surface preparations and thermal treatments on the composition of the near-surface region (∼7 atomic layers) and the topmost atomic layer of a polycrystalline Pd70Cu30 alloy. Palladium enrichment (relative to the bulk composition) is observed in the XPS-accessible near-surface region, but copper enrichment is observed in the topmost atomic layer. At temperatures above ∼800 K, where the bulk, the near-surface region and the topmost atomic layer are likely in thermodynamic equilibrium, segregation to the top layer can be described in terms of a simple thermodynamic model. Temperature programmed desorption (TPD) of H2 and CO from the annealed surfaces illustrates the impact of segregation and atomic distribution in the top layer on surface chemical activity.