Segregation at the surfaces of CuxPd1− x alloys in the presence of adsorbed S

a r t i c l e i n f o The influence of adsorbed S on surface segregation in Cu x Pd 1 − x alloys (S/Cu x Pd 1 − x) was characterized over a wide range of bulk alloy compositions (x = 0.05 to 0.95) using high-throughput Composition Spread Alloy Film (CSAF) sample libraries. Top-surface and near-surface compositions of the CSAFs were measured as functions of bulk Cu composition, x, and temperature using spatially resolved low energy ion scattering spectroscopy (LEISS) and X-ray photoemission spectroscopy (XPS). Preferential segregation of Cu to the top-surface of the S/Cu x Pd 1 − x CSAF was observed at all bulk compositions, x, but the extent of Cu segregation to the S/Cu x Pd 1 − x surface was lower than the Cu segregation to the surface of a clean Cu x Pd 1 − x CSAF, clear evidence of an S-induced " segregation reversal. " The Langmuir–McLean formulation of the Gibbs isotherm was used to estimate the enthalpy and entropy of Cu segregation to the top-surface, ΔH seg (x) and ΔS seg (x), at saturation sulfur coverages. While Cu segregation to the top-surface of the clean Cu x Pd 1 − x is exothermic (ΔH seg b 0) for all bulk Cu compositions, it is endothermic (ΔH seg > 0) for S/Cu x Pd 1 − x. Segregation to the S/Cu x Pd 1 − x surface is driven by entropy. Changes in segregation patterns that occur upon adsorption of S onto Cu x Pd 1 − x appear to be related to formation of energetically favored Pd\S bonds at the surface, which counterbalance the enthalpic driving forces for Cu segregation to the clean surface. The rates and mechanisms of processes that take place on the surfaces of multi-component materials, such as catalysis and corrosion, depend on surface composition. For multi-component materials, surface composition differs from bulk composition because of preferential segregation of one or more components to the surface [1–3]. Surface segregation minimizes the total free energy of the material; it is determined by bulk composition, environmental variables (temperature and pressure) and the presence adsorbed species on the surface [4–6]. An adsorbed species can interact preferentially with one or more of the component materials, resulting in " adsorbate-induced segregation " [7–9]. This phenomenon is of particular importance in the design of catalytic surfaces [10–15]; for example, adsorption of CO …