A defect-mediated direct synthesis of alane (AlH3) from its elements was predicted and verified experimentally. Alane has potential use as a hydrogen storage material with its capacity of 10.1 weight % H2 and relative ease of releasing hydrogen below 100 °C. Alane has not found broad practical use for over 70 years due to difficulties of direct synthesis from its elements. Using density functional theory researchers showed defect-mediated formation of alane monomers on crystalline faces of aluminum, namely Al(111), in a two-step process: (1) dissociative adsorption of H2 and (2) alane formation; both processes are endothermic (require heat) on clean surfaces. Adding small amounts of titanium facilitates H2 dissociation, and the presence of vacancies on the surface provides aluminum adatoms needed for the direct synthesis. With the Ti-dopant and vacancies, both processes become exothermic. In agreement, in situ scanning tunnelling microscopy showed that during H2 exposure alane monomers and clusters form primarily in the vicinity of Al vacancies and Ti atoms. Moreover, ball-milling samples of Al with Ti (to provide the necessary defects) showed a 10% conversion of Al into AlH3 or closely-related species at 344 bar H2, indicating that the predicted pathway leads to direct alane synthesis from the elements at pressures much lower than the 10,000 bar expected from bulk thermodynamics.
Predicted reaction energy path for AlH3 formation on Ti-doped Al(111)
Towards Direct Synthesis of Alane: A Predicted Defect-Mediated Pathway Confirmed Experimentally