We propose a reversible hydrogen storage mechanism for cycled sodium alanate. The individual mechanistic reaction steps for the decomposition reaction derive a set of time-dependent differential equations that simultaneously produce the evolution plots for each species. These plots reproduce several aspects of in situ X-ray diffraction (XRD) data, as well as the measured relative composition of selected decomposition samples at various extents of reaction. The presence of alanes facilitates both the decomposition and reformation of sodium alanate based on the principle of microscopic reversibility. A major role for the titanium dopant in catalyzed sodium alanate dynamics may be alloy formation at or near the surface of bulk aluminum that facilitates the formation and sorption properties of alanes. Published by Elsevier B.V.

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To ensure safe storage, plutonium-bearing oxides are stabilized at 950 °C for at least two hours in an oxidizing atmosphere. Stabilization conditions are expected to decompose organic impurities, convert metals to oxides, and result in moisture content below 0.5 wt%. During stabilization, the specific surface area is reduced, which minimizes readsorption of water onto the oxide surface. Plutonium oxides stabilized according to these criteria were sampled and analyzed to determine moisture content and surface area. In addition, samples were leached in water to identify water-soluble chloride impurity content. Results of these analyses for seven samples showed that the stabilization process produced low moisture materials (< 0.2 wt %) with low surface area (≤ 1 m 2 /g). For relatively pure materials, the amount of water per unit surface area corresponded to 1.5 to 3.5 molecular layers of water. For materials with chloride content > 360 ppm, the calculated amount of water per unit surface area increased with chloride content, indicating hydration of hygroscopic salts present in the impure PuO 2 -containing materials. The low moisture, low surface area materials in this study did not generate detectable hydrogen during storage of four or more years.

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...