Hydrogen desorption kinetics in transition metal modified NaAlH4

The compound NaAlH4 has been proposed as a viable hydrogen storage media capable of supplying hydrogen at moderate temperatures and at rates required for fuel cell applications. A number of researchers have subsequently verified and enhanced the rate of dehydrogenation in this system using combined transition metal catalytic additions along with different methods of their introduction into the NaAlH4 compound. The most potent catalytic addition identified thus far is TiCl3 added through ball milling, however, ZrCl3 and other transition metal chlorides have also been found useful. To this date, the role of the catalysts are still unknown as well as their disposition within the NaAlH4 compound. This investigation has sought to identify the extent catalytic dehydrogenation reaction rate of a number of transition metal additions to NaAlH4 at 120degreesC. Two mol% catalysts were added, primarily as chlorides, in a number of valance states. High energy ball mill attrition of the catalyst and the alanate were made with optimum milling times determined. Comparison of dehydrogenation rates with ion radius has shown an optimum catalyst ion radius ratio to be readily identifiable as being midway between that of Al(III) and Na(I). Cation valance, while an important factor in hydrogen discharge kinetics, was shown not to be the primary contributing factor for catalytic activity. (C) 2003 Elsevier B.V. All rights reserved.