| The storage of hydrogen into metals in the form of complex metal hydrides is one of the most promising methods. However, the incorporation and release of hydrogen requires very high temperatures. The discovery that the addition of Ti compounds lowers NaAlH4 decomposition barriers closer to ambient conditions, has re-ignited the field, and it is believed that surface processes are responsible for H2 dissociation and mass transport required to form the hydrogenated materials.;Such surface reactions mechanisms are however difficult to study with typical spectroscopic and imaging surface science tools. Alanes lack contrast under electron microscopes and can modify the Scanning Tunneling Microscopy (STM) tips. Infrared spectroscopy would be a sensitive probe to investigate the adsorption of hydrogen providing, but has so far failed to detect chemisorbed hydrogen on Ti-doped Al surfaces due to the weak Al-H dynamic dipole moment.;Thus despite extensive investigations, the fundamental mechanisms of the role of Ti and alane formation have remained elusive.;In this study combining surface infrared (IR) spectroscopy and density functional theory (DFT), we provide atomistic details about the role of Ti as a catalyst for hydrogen uptake and alane formation and evolution on single crystal Al(111) and Al(100) surfaces. We are able to detect H indirectly by using CO as a probe molecule of the weak Al-H species. We demonstrate that aluminum doped with very small amounts of titanium (in a specific configuration) can activate molecular hydrogen at temperatures as low as 90K. Once dissociated, hydrogen spills over from these catalytic sites on to the Al surface and protects the surface from further reactions.;We also show that, on Ti-doped Al surfaces, the diffusion dynamics are severely altered by Ti doping (Atomic hydrogen and AlH3 are trapped at the Ti sites) as indicated by a marked decrease of higher alane concentrations, which is deleterious for hydrogen storage for which mass transport is required. These results provide an understanding of the reaction mechanisms for hydrogen storage in the alanates (NaAlH4) that can help in identifying and realizing other complex metal hydrides for an effective hydrogen based economy. |
