Conventional (e.g. MgH 2 ) and complex hydrides (e.g. alanates, borohydrides, and amides) are the two primary classes of solid-state hydrogen-storage materials. [1][2][3] Many of these "high-density" hydrides have the potential to store large amounts of hydrogen by weight (up to 18.5 wt % for LiBH 4 ) and/or volume (up to 112 g L À1 for MgH 2 ), values that are comparable to the hydrogen content of gasoline (15.8 wt %, 112 g L À1 ). However, all known hydrides are inadequate for mobile storage applications due to one or more of the following limitations: a) unfavorable thermodynamics (they require high temperatures to release hydrogen [4] ), b) poor kinetics (low rates of hydrogen release and uptake), c) decomposition pathways involving the release of undesirable by-products (e.g. ammonia), and/or d) an inability to reabsorb hydrogen at modest temperatures and pressures (i.e. "irreversibility").In spite of these drawbacks, renewed interest in complex hydrides has been stimulated recently by substantial improvements in their kinetics and reversibility [5,6] provided by catalytic doping (e.g. TiCl 3 -doped NaAlH 4 ), [7,8] and by thermodynamic enhancements achieved through reactive binary mixtures [9] such as LiNH 2 /MgH 2 , [10,11] LiBH 4 /MgH 2 , [12] and LiNH 2 /LiBH 4 . [13,14] These compositions, previously termed "reactive hydride composites", [15] represent the state-of-the-art in hydrogen-storage materials; compared to their constituent compounds, they exhibit improved thermodynamic properties, higher hydrogen purity, and, in some cases, reversibility. The desorption behavior of these previously studied composites is illustrated in Figure 1 a. It is evident from the hydrogen desorption profile (top panel) that the composites generally desorb hydrogen at significantly lower temperatures than their individual components. For example, the lowest temperature reaction, which involves a Figure 1. a) Hydrogen (top) and ammonia (bottom) kinetic desorption data as a function of temperature (5 8C min À1 to 550 8C) for the ternary composition (blue trace) and its unary and binary constituents. Hydrogen desorption is measured in weight percent (wt %) to 1 bar whereas relative ammonia release is measured as partial pressure (torr) in a flow-through set-up (100 sccm Ar). b) Ternary phase space defined by unary compounds (nodes), LiBH 4 (pink), MgH 2 (purple), and LiNH 2 (orange) and the binary mixtures (edges), LiBH 4 /MgH 2 (gray), MgH 2 / LiNH 2 (green), and LiNH 2 /LiBH 4 (red). The present ternary composition, which is a 2:1:1 mixture of LiNH 2 , LiBH 4 , and MgH 2 , and previously investigated binaries, are identified.

Epitaxial Au layers were prepared by vapor deposition of Au on a Pt (111) single crystal surface. Surface alloys were formed by heating the Pt(lll) crys-•"tal that was covered with a Au multilayer. The surfaces were characterized with AES, LEED and TPD of CO. LEED showed that both gold over layers and Pt-Au alloy surfaces were well ordered and had (1 x 1) surface structures. The temperature at the maximum of the TPD spectrum of CO was found to be sensitive to the distribution of Pt atoms in the surface. As a result, epitaxial surfaces were shown to have large Pt ensembles while much smaller ensembles were predominant on alloy surfaces. With TPD of CO it could also be demonstrated that the ligand effect of alloying is absent for CO adsorbed on these alloy surfaces. The differences in ensemble size were found to have profound effects on the skeletal reactions of n-hexane. This reaction was carried out in situ with 26.7 mbar (20 Torr) n-hexane in 267 mbar (200 Torr) hydrogen at 573 K, by utilizing a sample isolation cell.Surface alloys were found to be more active than pure Pt( 111). Large increases in the isomerization rate of n-hexane and simultaneous exponential decreases of hydrogenolysis and aromatization rates with gold coverage led to high selectivity for isomerization. Changes in the bonding of organic molecules must be responsible for this effect that are induced by structural alterations of the (111) orientation platinum crystal surface. Epitaxial gold layers decreased the activity of the Pt(111) surface in proportion to the gold coverage because of the reduction of the available platinum surface area, without substantial selectivity changes.

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