Microwave irradiation effects on reversible hydrogen desorption in sodium aluminum hydrides (NaAlH₄)

The effect of microwave irradiation on the reversible desorption reaction in sodium aluminum hydride (NaAlH₄) is explored. NaAlH₄ is doped with 2 mol% TiCl₂ and pre-activated by high energy ball milling and aging to show the presence of metallic aluminum phase. As a catalyst, Ti²⁺ has been used to improve desorption kinetics in sodium alanate. X-ray diffraction was performed on the samples exposed to microwave irradiation for 10, 20, 30, 40 and 50 min. Results show that when the powders show the presence of aluminum, a steady increase in the formation of the hexahydride (Na₃AlH₆) phase and Al occurs during microwave irradiation; and is accompanied by a steady reduction in the NaAlH₄ phase XRD peak (h k l) intensities. This data suggests that microwave irradiation drives the reversible H₂ desorption reaction in NaAlH₄. NaAlH₄ doped with 2 mol% TiCl₂ which does not show the presence of Al phase, undergoes a reduction in NaAlH₄ peak intensities with increasing microwave exposure (and no reversible product phases are detected in this case). Dielectric studies on NaAlH₄ indicate that microwave penetration is low. Therefore, it is proposed that microwave irradiation heating of the Al particulate phase is responsible for the hydrogen desorption reaction pathway which is similar to that of conventional heating.