(392e) 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 (3d_3/2); the
feature at 156 is characteristic of reduced Y (3d_5/2); the feature at ~158 eV
is likely a combination of Y-oxide (3d_5/2)
and reduced Y (3d_3/2).  Y exists as oxide only at top-surface, before
sputtering begins.