Synthesis of calcium alanate and its dehydriding performance enhanced by FeF 3 doping

Abstract

Ca(AlH 4) 2 was synthesized by ball-milling the mixture of NaAlH 4 and CaCl 2 in a molar ratio of 2:1 and under a hydrogen atmosphere of 1 MPa. The results indicate that the reactants have entirely transformed to Ca(AlH 4) 2 with a byproduct of NaCl after ball-milling for 48 h. Investigations of dehydriding behavior of the as-prepared Ca(AlH 4) 2 sample show that approximately 5.2 wt.% of hydrogen is desorbed during the first two dehydrogenation reactions of Ca(AlH 4) 2, which exhibit an exothermic event at 148 °C and an endothermic event at 267 °C, respectively. The high temperature dehydrogenation at 267 °C mainly concentrates on the thermolysis of CaAlH 5 intermediate. FeF 3-doped Ca(AlH 4) 2 system represents an improved dehydriding performance, the dehydrogenation temperature of CaAlH 5 intermediate is decreased about 43 °C. After FeF 3 doping, the apparent activation energy of CaAlH 5 is reduced from 153.4 kJ/mol (undoped) to 88.3 kJ/mol (doped), it renders a possibility to realize the rehydrogenation of CaAlH 5. The catalytic effect is attributed to a fluorine transfer reaction that occurred to generate CaF 2 and Fe catalysts.