A general initial decomposition reaction for complex metal hydrides

Abstract

The initial thermally activated decomposition of several complex metal hydride compounds, to a binary alkali or alkaline hydride and a group IIIb metal hydride, appears to share a first step in their decomposition mechanisms. The application of this initial thermochemical decomposition step to several alanate compounds illustrates the generality of this approach. For LiAlH 4 , the decomposition data fall on the derived distribution plot calculated for NaAlH 4 . INTRODUCTIONMany enthusiastic efforts continue to contribute to a solid-state reversible hydrogen storage system that will enable an increased participation in an emerging hydrogen economy [1]. While the use of hydrogen as an energy carrier reduces our dependence on petroleum, solid-state hydrogen storage looms on the energy horizon as a major technical concern. Nevertheless, a number of complex metal hydride materials of composition A m (MH x ) n , where A is an alkali or alkaline earth metal (Li, Na, Mg, K), and M is a group IIIb metal (B, Al), hold the promise of high weight percent, solid-state hydrogen storage. Examples of these materials under investigation for hydrogen storage properties are NaAlH 4 and Na 3 AlH 6 , LiAlH 4 and Li 3 AlH 6 , Na 2 LiAlH 6 , Mg(AlH 4 ) 2 , KAlH 4 and K 3 AlH 6 , and LiBH 4 [2][3][4][5][6]. Although none has emerged as a solid storage panacea, recent advances in transition metal doping have imparted attractive low temperature, reversible, solid-state hydrogen storage properties to these complex metal hydrides [1].A key to deploying a solid-state hydrogen storage system lies in understanding the fundamental thermochemical processes intrinsic to hydride storage materials. Of late, we proposed a reversible solid-state hydrogen storage mechanism for NaAlH 4 [7]. Central to this mechanism is the initial production of a binary alkali hydride (NaH) and an intermediate alane (AlH 3 ) that migrates and delivers the evolved hydrogen gas upon decomposition at a titaniumaltered (catalytic) aluminum surface [8]. Then, by the application of high-pressure hydrogen, the same alane species seeks out NaH and produces the original alanate, thus accounting for the reformation of crystalline NaAlH 4 . The initial thermally activated decomposition step, producing a binary alkali or alkaline hydride and a group IIIb metal hydride, appears to be a general first step in the decomposition mechanism for this class of complex metal hydride compounds.The purpose of this short note is to point out that the application of this initial decomposition step to alkali and alkaline alanates demonstrates how some of the accumulating thermo-kinetic data now appearing in the literature for these potentially important solid-state hydrogen storage materials can begin to be rationalized. The proposed shared complex metal hydride initial decomposition step is reaction (1). This reaction initiates an autocatalytic reaction network that continues the decomposition. The reversible capacity of the complex metal hydride, therefore, derives from the decomposit...

Investigations on the desorption kinetics of Mm-doped NaAlH4

“…As regards the role of catalyst Mm in enhancing the desorption kinetics, and in improving the reversibility it may be opportune to first recapitulate the case of catalyst Ti which has widely been studied by several workers [2][3][4][5][6][7][11][12][13][14][15][16][17][18][19][20][21][28][29][30][31]. In spite of extensive works, the location and exact role of Ti has not yet become clear.…”

“…Various studies have been carried out on Ti doped and bare alanates [11][12][13][14][15] for analyzing the location and role of the Ti catalyst. However, these studies have not been able to elucidate the said aspects of the catalyst Ti [2][3][4][5][6][7][8][9][10][16][17][18][19][20][21][22][23][24][25][28][29][30][31]. Besides transition metals Ti, Zr and Fe, carbon nanotubes have also been found to enhance hydrogen desorption in NaAlH 4 [24,25].…”