Hydrogen interaction with Al(111) and Ti-doped Al(111) surfaces for H storage applications

Complex metal hydrides are promising candidates for hydrogen storage applications. Most of these materials consist of aluminum mixed with other elements. It was found that titanium doping allows their use in a convenient temperature and pressure range. Therefore the key process is the dissociation of molecular hydrogen at a Ti catalyst on the surface. Preliminary studies with pure and titanium-doped aluminum surfaces are undertaken to gain a basic understanding of the hydrogen adsorption mechanism. The interaction of hydrogen with an Al(111) surface was studied using Infrared Reflection Absorption Spectroscopy (IRRAS) under Ultra High Vacuum (UHV) conditions. Hereby the formation of aluminum hydrides (alanes) in different sizes was observed depending on the hydrogen coverage of the surface. The growth of larger alanes is favored at higher H exposures. Different measurements were carried out for temperatures of 93, 180 and 259K and it was found that larger alanes form at higher temperatures. In another step of the experiment an Al surface was doped with titanium atoms emitted from a home-built Ti-source. Ti structures with a thickness of less than one monolayer (ML) were grown at 105K and a Ti film of around one ML was deposited at room temperature. The deposition rate was determined with a quartz crystal monitor and the deposition on the surface could be verified by Auger Electron Spectroscopy (AES). Low Energy Electron Diffraction (LEED) patterns were recorded before and after different Ti-depositions and were always showing the typical hexagonal symmetry for the closed packed (111) surface of the fcc Al crystal. To compare the vibrational modes of alanes forming on Al(111) surfaces with the ones of solid state AlH3 Fourier Transform Infrared Spectroscopy (FTIR) was performed in a different setup using a high vacuum. No matches of the frequencies could be found but the decomposition of AlH3 to Al and H could be observed at temperatures higher than 170°C.M.S.Includes bibliographical references (p. 74-76)