280373 Segregation At the Surfaces of Cu8Pd7M Hydrogen Separation Alloys in the Presence of Adsorbed O and S
The CuPd B2 phase is stable only at relatively low temperatures; at high temperatures, it undergoes a phase transformation that reduces its hydrogen permeability. We have recently identified a set of alloying elements (Y, Al, and Mg) that can stabilize the B2 phase of the copper-palladium alloy by expanding its composition-temperature phase field. In this work, we examine surface segregation of these elements in the CuPd B2 lattice using both density functional theory calculations and experimental measurements. Trends in segregation, adsorption, and surface free energies are characterized. Theoretical results show that in vacuum, Y will substitute first for Cu and then for Pd at the sub-surface lattice site before segregating to the surface where it substitutes for Cu. However, in air, Y preferentially occupies surface sites due to its stronger oxygen affinity compared to Cu and Pd. XPS experiments reveal that surface segregation of Y is induced by formation of Y-oxides at the top-surface of the alloy. Preliminary XPS results for a Cu-Pd-Al alloy that had been exposed to H2S show that, while adsorbed S does not penetrate far into the alloy, it induces significant segregation of Cu to the alloy surface at the expense of Pd. In the case of Cu-Pd-Mg, adsorbed S leads to strong segregation of Mg to the alloy surface at the expense of both Pd and Cu.
Figure 1: Y3d region of the XPS depth profile. The feature at ~160 eV is characteristic of Y-oxides (3d3/2); the feature at 156 is characteristic of reduced Y (3d5/2); the feature at ~158 eV is likely a combination of Y-oxide (3d5/2) and reduced Y (3d3/2). Y exists as oxide only at top-surface, before sputtering begins.