Hydrogen exchange kinetics in NaAlH4 catalyzed in different decomposition states

Hydrogen absorption and desorption kinetics of NaAlH4 catalyzed with small Ti-clusters were studied. Three different kinds of samples were investigated based on undecomposed NaAlH4, Na3AlH6, and NaH+Al, the latter two both produced by decomposition of pure NaAlH4 and mechanical alloying. The samples, catalytically doped in the described decomposition state, showed different absorption kinetics and storage capacities. Whereas the same kinetic behavior was observed for the first decomposition step, the second step exhibited strong differences. The absorption behavior of the samples showing the fastest kinetics and highest storage capacities was further investigated by absorbing hydrogen in two separate steps. The first decomposition/absorption step can be well interpreted with a nucleation and growth model.