| dc.description.abstract | Hydrogen production from carbon-containing energy sources can capture the CO2 on site,
and therefore reduce the contribution to global warming. The only emission when energy
is produced through a fuel cell is H2O. Another advantage is that hydrogen is an energy
carrier and can be stored over a long time period. Palladium membranes are 100 % selective
to hydrogen, it is interesting to look at palladium alloys as they can improve some of the
disadvantages with pure palladium. Also, a model system of membranes, a single crystal,
can be studied to get a better understanding of the physical and chemical behavior.
Surface investigations, permeation and solubility measurements of a PdAu8wt% membrane
(8 μm) grown by magnetron sputtering provided by Sintef have been performed. Other
membranes like Pd, PdAg and PdAgAu have also been studied for comparison in some of
the experiments.
Surface investigations in X-ray photoelectron spectroscopy (XPS) have been executed for Pd,
PdAg, PdAu and PdAgAu membranes. Comparison of chemical shifts compared to pure Pd
showed PdAg and PdAgAu shifts to lower binding energy, but PdAu was almost the same
as Pd. The subpeak contributions of oxygen was found at higher binding energies about
0.5-0.8 ev (highest intensity) and 1.3-1.9 eV relative to the Pd 3d5/2 bulk peak. Oxygen
induced peak were found for palladium, but not silver and gold in XPS. This indicates that
oxidation only is occuring at palladium. Gold and silver were found to segregate to the
surface in the PdAu and PdAg membranes investigated. For the ternary alloy PdAgAu only
silver segregates.
Surface topography investigations were performed in a scanning electron microscope (SEM).
The surface on the growth side of the membrane was rough compared to the substrate side.
The growth side surface is polycrystalline with cone-shaped features of different size up to
about 2.2 μm.
To investigate the membrane properties, solubility and permeation experiments were performed.
Solubilitymeasurements were performed to find the Sievert s constant. The Sievert s
constant for the PdAu membrane was found to be higher than reported earlier in literature
for similar membranes. The permeability was found to increase with heat treatment in air
(HTA) with an improvement of about 21-33 %. Some uncertainties in the measurements
were found as different permeabilities were found for the same type of membrane in the two
parallels. The permeabilities were found to increase with temperature. Permeability also
showed higher dependency on diffusion than of solubility.
Surface investigations of the single surface crystal Pd3Au(100) has been performed in Scanning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction (LEED). The experiments were performed under ultra-high vacuum (UHV) conditions to easier attain high
resolution with less contaminants at atomic scale. Both the clean surface and the surface
oxide structure of Pd3Au(100) have been investigated. A unit cell with proportions p(1×1)
was found for the clean surface with investigations in both LEED and STM. The surface
oxide structure of Pd3Au(100) was found to be (p5 × p5)R27o in LEED. | en |
