The researchers found that atomic-level defects, when present on the surface of aluminum together with hydrogen and titanium, created the conditions necessary for a chemical reaction producing alane to occur, and without the need for hydrogen at tremendously high pressure. While not ready for mainstream synthesis, this work serves as proof of concept of a more efficient and less costly way of producing alane.

The researchers paired the predictive advantages of computational analysis with physical experiment to tackle the applied materials challenge. Along with titanium catalyst dopants and hydrogen, theorists looked at vacancy defects, or missing aluminum, on the surface of aluminum powders and established that this combination working in concert is critical to the low-energy formation of alane.

Because such defects can be produced by ball milling to break up mechanically the atomic structure of the metal, experimentalists ball-milled aluminum powders in combination with hydrogen and titanium, and they confirmed the prediction by producing alane. The process used significantly less pressure, only about 5,000 PSI (or 30 times less pressure), to create alane than that needed for equilibrium methods.

Using density functional theory (DFT) calculations, we studied the formation of alane monomers on Al(111) surfaces involving the cooperative effects of defects (surface vacancies and catalyst) in a two-step reaction: (1) dissociative adsorption of H2 and (2) alane formation from Al adatoms and atomic H. On pristine Al(111), both steps are endothermic, whereas Step 1 becomes exothermic if Al(111) is activated with either vacancies or Ti dopant. However, if Al(111) is activated with both Ti dopant and vacancies to facilitate H2 dissociation and provide Al adatoms, respectively, it becomes thermodynamically favorable to form alane monomers in both steps of the surface reaction.

Alane formation on the Al(111) surface during exposure to molecular hydrogen was observed in situ by scanning tunneling microscopy (STM), mainly at locations where Ti dopant atoms and Al vacancies were both present. The predicted co-operative point-defect-mediated route explains our STM observations and predicts the feasibility of direct mechanochemical hydrogenation of Al. A preliminary mechanochemical experiment shows approximately 10% conversion of Al into AlH₃ or closely related species at 344 bar H₂ in the presence of catalytic Ti, a pressure much lower than the 104 bar expected from bulk thermodynamics.

—Wang et al.

Through the mechanochemistry you create as many vacancies as you can in a powder, which increases the surface area, and the process yields 10 percent alane. Alane is light, when it gives up hydrogen and transforms to aluminum it becomes recyclable like aluminum cans, it’s under no pressure, and therefore is safe for vehicle applications. While 10 percent is not yet a commercially viable yield, the science here puts it within reach upon further research and development.

—Duane Johnson, Ames Laboratory Chief Research Officer and F. Wendell Miller Professor of Energy Science at Iowa State University’s department of Materials Science and Engineering

Resources

• L.-L. Wang, A. Herwadkar, J. M. Reich, D. D. Johnson, S. D. House, P. Peña-Martin, A. A. Rockett, I. M. Robertson, S. Gupta, V. K. Pecharsky (2016) “Towards Direct Synthesis of Alane: A Predicted Defect-Mediated Pathway Confirmed Experimentally” ChemSusChem 9, 2358 doi: 10.1002/cssc.201600338

• Thanh Q. Hua, Rajesh K. Ahluwalia (2011) “Alane hydrogen storage for automotive fuel cells – Off-board regeneration processes and efficiencies,” International Journal of Hydrogen Energy, Volume 36, Issue 23, Pages 15259-15265 doi: 10.1016/j.ijhydene.2011.08.081