Mechanisms for the enhanced hydrogen desorption performance of the TiF4-catalyzed Na2LiAlH6 used for hydrogen storage

A dramatic enhancement in the dehydrogenation/hydrogenation kinetics of Na2LiAlH6 was achieved by adding 5 wt% TiF4. The starting temperature for dehydrogenation of the sample with TiF4 added was lowered by 50 degrees C with respect to the pristine sample. In-depth investigations revealed that the addition of TiF4 to Na2LiAlH6 not only enhanced the dehydrogenation kinetics, but also altered the dehydrogenation thermodynamics. The apparent activation energy and the desorption enthalpy change of the Na2LiAlH6 with 5 wt% TiF4 added were calculated to be ca. 143.6 +/- 2.7 kJ mol(-1) and 57.3 +/- 2.3 kJ mol(-1) of H-2 by using Kissinger's approach and the van't Hoff equation, both lower than those of the pristine sample, respectively. We firmly believe that both Ti and F play critical roles in the dehydrogenation/hydrogenation process of the Na2LiAlH6 sample with added TiF4 due to the formation of Ti-Al clusters, the substitution of F- for H- and the presence of LiF. All these are responsible for the significant improvement of hydrogen storage performances of the Na2LiAlH6 sample with the addition of TiF4. This understanding provides us with insights into the dehydrogenation kinetic mechanisms of Na2LiAlH6 used for hydrogen storage and directs the optimization of high-performance catalysts for the hydrogen storage reaction of the alanates.