One step high pressure mechanochemical synthesis of reversible alanates NaAlH4 and KAlH4

• NaAlH4 & KAlH4 were synthesized by high pressure milling technique.
• High pressure mechanochemical synthesis is better than THF activated synthesis.
• H2 desorption at ∼100 °C is observed in both NaAlH4 & KAlH4 samples.
• Catalyzed samples were purified and the extracts were studied.
• Extract contains Ti–Al alloy and finely dispersed NaCl nanoparticles.

The present study suggests high pressure mechanochemical treatment is a better strategy for the synthesis of performance enhanced reversible alanates. The reactants NaH/KH + Al along with catalysts (TiCl3, TiF3 and TiO2) milled under 100 bar hydrogen pressure for 30 h effectively transforms to products (NaAlH4 and KAlH4) in a single step. The as-synthesized NaAlH4 and KAlH4 samples release hydrogen at the temperatures of ∼100 °C and 215 °C, respectively. The stability of the KAlH4 phase can be further reduced by extending the high pressure mechanochemical reaction time to 80 h. The XRD and TEM analysis of the residues observed after extracting the NaAlH4 from the TiCl3 catalyzed material confirms the presence of Ti–Al alloy and highly dispersed NaCl nanoparticles. Catalytic activity is therefore attributed to mechano-chemical activation which involves catalytically active species (for e.g. Ti–Al) and defects/vacancies/strain in the system.