Direct synthesis of sodium alanate using mischmetal nanocatalyst

This study reports the synthesis of NaAlH4 by ball milling of NaH and Al mixture along with 3 mol % Mischmetal (Mm) nanocatalyst under hydrogen atmosphere. It is observed that synthesis of the intermediate phase Na3AlH6 can be achieved by ball milling even under 1 atm hydrogen at room temperature. Ball milling of the NaH + Al with 3 mol % Mm with 3 atm hydrogen in excess of 40 h time did not lead to the formation of NaAlH4 but charging of the milled material at 100 atm hydrogen pressure at 120 °C lead to formation of NaAlH4 phase. Direct synthesis of NaAlH4 was achieved by milling of NaH + Al with 3 mol % Mm under 100 atm hydrogen pressure. Direct synthesis is possible even without any catalyst by high pressure milling. However catalyst is required to improve the hydrogen sorption characteristics of the synthesized material. The as-prepared Mm catalyzed NaAlH4 is also found to reversibly store hydrogen up to 4.2 wt% hydrogen. Catalytic activity is attributed to defects promoted by ball milling and catalysts.

► NaH and Al as reactants and Mm as catalyst for the ab-initio synthesis of NaAlH4. ► A two step synthesis and one step direct synthesis methods-both are mechanical milling routes. ► High pressure milling of reactants under 100 atm hydrogen pressure. ► Na3AlH6 formation under STP conditions and NaAlH4 formation under higher pressure.