Investigations of PdAu membranes and model systems for production of pure hydrogen
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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 energyis produced through a fuel cell is H2O. Another advantage is that hydrogen is an energycarrier and can be stored over a long time period. Palladium membranes are 100 % selectiveto hydrogen, it is interesting to look at palladium alloys as they can improve some of thedisadvantages 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. Othermembranes like Pd, PdAg and PdAgAu have also been studied for comparison in some ofthe 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 Pdshowed PdAg and PdAgAu shifts to lower binding energy, but PdAu was almost the sameas Pd. The subpeak contributions of oxygen was found at higher binding energies about0.5-0.8 ev (highest intensity) and 1.3-1.9 eV relative to the Pd 3d5/2 bulk peak. Oxygeninduced peak were found for palladium, but not silver and gold in XPS. This indicates thatoxidation only is occuring at palladium. Gold and silver were found to segregate to thesurface in the PdAu and PdAg membranes investigated. For the ternary alloy PdAgAu onlysilver 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 toabout 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 sconstant for the PdAu membrane was found to be higher than reported earlier in literaturefor 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 measurementswere found as different permeabilities were found for the same type of membrane in the twoparallels. The permeabilities were found to increase with temperature. Permeability alsoshowed 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 highresolution with less contaminants at atomic scale. Both the clean surface and the surfaceoxide 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 surfaceoxide structure of Pd3Au(100) was found to be (p5 × p5)R27o in LEED.