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Molten mixtures of ytterbium and lithium metals can be used as fluxes for the synthesis of metal hydrides. Reactions of indium and lithium hydride in Yb/Li flux produce Yb~51In13H27 (cubic, Im-3; a = 16.218(6) A, Z = 2). Yb~51In13H27 is an analog of previously reported Ca54In13H27 which was grown from Ca/Li flux. The compound features In@Yb12@H20@Yb30 Bergman-like clusters at the corners and body centers of the cubic lattice. The clusters are connected by a hydride site and disordered ytterbium sites. Magnetic susceptibility measurements indicate the ytterbium is divalent; the compound shows diamagnetic behavior with a Curie tail at lower temperatures attributed to impurities such as traces of Yb3+ that form on crystal surfaces due to slight oxidation.

ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H- could supply. Polarized, electron-rich NxHy intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N2, NH3, and H2, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH3. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N2 is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H- offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.

Experimental and theoretical studies were conducted to investigate the pyrophoricity and water-reactivity risks associated with employing sodium alanate (NaAlH4) complex metal hydride in on-board vehicular hydrogen (H2) storage systems. The ignition and explosivity of NaAlH4 upon exposure to oxidizers in air or water were attributed to the spontaneous formation of stable hydroperoxyl intermediates on the NaAlH4 surface and/or H2 production, as well as the large driving force for NaAlH4 conversion to favorable hydroxide products predicted by atomic and thermodynamic modeling. The major products from NaAlH4 exposure to air: NaAl(OH)4, gibbsite and bayerite Al(OH)3, and Na2CO3 observed by XRD, were identified to be formed by surface-controlled reactions. The reactivity risks were significantly minimized, without compromising de-/re-hydrogenation cyclability, by compacting NaAlH4 powder into wafers to reduce the available surface area. These core findings are of significance to risk mitigation and H2 safety code and standard development for the safe use of NaAlH4 for on-board H2 storage in light-duty vehicles.

Nanoscale CeAl4 was directly synthesized by the thermal reaction between CeH2 and nano-aluminum at 300 °C. Then nano CeAl4-doped sodium alanate (NaAlH4) was synthesized by ball milling NaH/Al with 0.04CeAl4 under hydrogen atmosphere at room temperature, and the catalytic efficiency of nanoscale CeAl4 for hydrogen storage of NaAlH4 was systematically investigated. It is shown that CeAl4 can effectively improve the dehydrogenation properties of sodium alanate system. The 0.04CeAl4-doped NaAlH4 system starts to release hydrogen below 80 °C, completes dehydrogenation within 10 min at 170 °C, and exhibits good cycling de/hydrogenation kinetics at relatively lower temperature (100–140 °C). Apparent activation energy of the dehydrogenation of NaAlH4 can be effectively reduced by addition of CeAl4, resulting in the decrease in desorption temperatures. Moreover, by analyzing the reaction kinetics of nano CeAl4-doped NaAlH4 sample, both of the decomposition steps are conformed to a two-dimensional phase-boundary growth mechanism. The mechanistic investigations gained here can help to understand the de-/rehydrogenation behaviors of catalyzed complex metal hydride systems. Nanoscale CeAl4 was successfully synthesized and doped into NaAlH4 system ball milling NaH/Al + 0.04CeAl4. The addition of CeAl4 can effectively improve the dehydrogenation properties of NaAIH4, which starts to release hydrogen below 80 °C, completes dehydrogenation within 10 min at 170 °C, and exhibits good cycling de/hydrogenation kinetics at relatively lower temperature.

An empirical study of the gas–solid reaction of carbon dioxide (CO2) with alanates is presented. This investigation was triggered by reports of hazards related to the reaction of lithium aluminum hydride with carbon dioxide, together with the recent emergence of alanates as potential hydrogen storage materials. Furthermore, the reduction of CO2 by hydrides is an alternative to the conventional CO2 reduction employing hydrogen gas in combination with a catalyst. Experimentally this work was carried out by studying the decomposition of alanate samples in a carbon dioxide atmosphere with thermogravimetric and ex situ IR spectroscopic techniques. It is shown that alanates react with CO2 at atmospheric pressure in two distinct temperature regions, yielding methane, hydrogen gas, and metal oxides as the major products. The experimental findings allowed us to postulate a mechanism for the complex metal hydride reduction of CO2, involving alane as a highly reactive intermediate. It is suggested that nucleophilic ...

Sodium aluminum hydride (NaAlH4) was directly synthesized by ball milling NaH/Al co-doped with CeCl3 + KH under a hydrogen pressure of 3 MPa at room temperature. Out of various samples corresponding to xNaH/Al + 0.02CeCl3 + yKH (x + y = 1; y = 0, 0.02, 0.04 mol%) composites, the composite with y = 0.02 exhibits the optimum de/hydrogenation properties. It shows that the addition of KH can effectively improve the dehydrogenation properties of second step reaction of NaAIH4 system. The composite with y = 0.02 starts to release hydrogen from 87 °C and completes dehydrogenation within 20 min at 170 °C, with good cycling de/hydrogenation kinetics at relatively lower temperature (100–140 °C). After ball milling, the CeCl3 precursor can be changed into CeH2 catalytic active component in the first several de/hydrogenation cycles. Apparent activation energy of the second decomposition step of NaAIH4 system can be effectively decreased by addition of KH, resulting in the decrease of desorption temperatures. Based on the microstructure analyses combined with hydrogen storage performances, the improved dehydrogenation properties of sodium aluminum hydride system are ascribed to the lattice volume expansion of Na3AlH6 during the dehydrogenation process resulted from the addition of KH. Moreover, by analyzing the reaction kinetics of CeCl3 + KH co-doped sample, both of the decomposition steps of composite with y = 0.02 were conformed to the two-dimension phase-boundary growth mechanism. The mechanistic investigations gained here could help to understand the de/rehydrogenation behaviors of catalyzed complex metal hydride systems.

Complex metal hydride is a promising hydrogen storage material for automobile applications due to its reversible storage capacity. The presence of transition metal halide is found to improve significantly the kinetics of H2 adsorption and desorption processes. Experimental studies have indicated the formation of distorted MAl3 phase where M = Sc, Ti, Zr. In this study, a first-principles density functional study has been performed to investigate the hydrogen interaction and saturation on stable tetrahedral MAl3 clusters [M = Li, Sc, Ti, and Zr] by employing spin-polarized hybrid and non-local density functionals. On saturation, the first H2 molecule undergoes chemisorption in the transition metals while further loading results in physisorption with the Kubas-type H2 interaction. Activation energy barrier for the H2 dissociation over the cluster is calculated to be ∼0.2 eV for the transition metals. Effect of external electric field on the MAl3H4 cluster with molecular H2 is studied which leads to polarization of physisorbed H2 and the cluster. The results offer an explanation for catalysts role in improving the kinetics of H2 sorption process in complex metal hydrides.

Zintl phase compounds constitute a unique class of compounds composed of metal cations and covalently bonded multiply charged cluster anions. Potential applications of these materials in solution chemistry and thermoelectric materials have given rise to renewed interest in the search for new Zintl ions. Up to now these ions have been mostly composed of group 13, 14, and 15 post-transition metal elements and no Zintl ions composed of all transition metal elements are known. Using gradient corrected density functional theory we show that the 18-electron rule can be applied to design a new class of Zintl-like ions composed of all transition metal atoms. We demonstrate this possibility by using Ti@Au12(2-) and Ni@Au6(2-) di-anions as examples of Zintl-like ions. Predictive capability of our approach is demonstrated by showing that FeH6(4-) in an already synthesized complex metal hydride, Mg2FeH6, is a Zintl-like ion, satisfying the 18-electron rule. We also show that novel Zintl phase compounds can be formed by using all transition metal Zintl-like ions as building blocks. For example, a two-dimensional periodic structure of Na2[Ti@Au12] is semiconducting and nonmagnetic while a one-dimensional periodic structure of Mg[Ti@Au12] is metallic and ferromagnetic. Our results open the door to the design and synthesis of a new class of Zintl-like ions and compounds with potential for applications.

Concentrating solar–thermal power (CSP) with thermal energy storage (TES) represents an attractive alternative to conventional fossil fuels for base-load power generation. Sodium alanate (NaAlH4) is a well-known sodium-based complex metal hydride but, more recently, high-temperature sodium-based complex metal hydrides have been considered for TES. This review considers the current state of the art for NaH, NaMgH3−x F x , Na-based transition metal hydrides, NaBH4 and Na3AlH6 for TES and heat pumping applications. These metal hydrides have a number of advantages over other classes of heat storage materials such as high thermal energy storage capacity, low volume, relatively low cost and a wide range of operating temperatures (100 °C to more than 650 °C). Potential safety issues associated with the use of high-temperature sodium-based hydrides are also addressed.

Li-N-H system, as a representative of metal-N-H system, is regarded as one of the most promising hydrogen storage materials. Motivated by the ammonia mediated mechanism of Li-N-H system, the effect of MgCl2 (which is an effective NH3 sorbent) on this syst

Carbonaceous nanomaterials utilized as scaffolds, catalysts, and additives in conjunction with complex metal hydrides have shown remarkable hydrogen sorption properties. Our studies have confirmed fullerene-C 60 is an excellent catalyst for temperature induced hydrogen desorption for both NaAlH 4 and LiAlH 4 . Fullerene-containing complex metal hydride composites comprised of fullerene-C 60 with NaAlH 4 or LiAlH 4 desorbed hydrogen at elevated temperature and go onto form alkali metal fullerides and aluminum metal as final products. The as-prepared composites exhibit rapid hydrogen desorption at onset temperatures of 130 °C and 150 °C, and released hydrogen content of 5.9 and 2.2 wt.% (LiAlH 4 and NaAlH 4 , respectively) relative to the composite. The resultant alkali metal fulleride containing composites have been characterized and are capable of reversible hydrogen storage. A series of desorption/absorption experiments on the Na–C 60 and Li–C 60 based composites demonstrate a 1.5 wt.% and a 1.2 wt.% reversible capacity, respectively. The complex metal hydride-C 60 systems were characterized by PCT, XRD, FT-IR, and TGA-RGA and demonstrate the formation of fulleride material similar to traditional hydrofullerenes which appear to be responsible for the observed reversible hydrogen storage.

Complex metal hydrides such as sodium aluminohydride (NaAlH4) and sodium borohydride (NaBH4) are solid-state hydrogen-storage materials with high hydrogen capacities. They can be used in combination with fuel cells as a hydrogen source thus enabling longer operation times compared with classical metal hydrides. The most important point for a wide application of these materials is the reversibility under moderate technical conditions. At present, only NaAlH4 has favorable thermodynamic properties and can be employed as a thermally reversible means of hydrogen storage. By contrast, NaBH4 is a typical non-reversible complex metal hydride; it reacts with water to produce hydrogen.

A novel metal borohydride ammonia-borane complex Ca(BH4)2·NH3BH3 is characterized as the decomposition product of the recently reported perovskite-type metal borohydride NH4Ca(BH4)3, suggesting that ammonium-based metal borohydrides release hydrogen gas via ammonia-borane-complexes. For the first time the concept of proton-hydride interactions to promote hydrogen release is applied to a cation-anion pair in a complex metal hydride. NH4Ca(BH4)3 is prepared mechanochemically from Ca(BH4)2 and NH4Cl as well as NH4BH4 following two different protocols, where the synthesis procedures are modified in the latter to solvent-based ball-milling using diethyl ether to maximize the phase yield in chlorine-free samples. During decomposition of NH4Ca(BH4)3 pure H2 is released, prior to the decomposition of the complex to its constituents. As opposed to a previously reported adduct between Ca(BH4)2 and NH3BH3, the present complex is described as NH3BH3-stuffed α-Ca(BH4)2.

Magnesium alanate Mg(AlH4)2 has been synthesized by mechanochemically activated metathesis reaction between NaAlH4 and MgCl2 without solvent, this metathesis reaction proceeded completely. The thermal decomposition behavior of solvent-free Mg(AlH4)2 has been analyzed by TG/MS and DSC. It has been confirmed that Mg(AlH4)2 decomposes in two steps: the first decomposition reaction Mg(AlH4)2 → MgH2 + 2Al + 3H2 is exothermic and starts at about 115 °C. The second one Mg H 2 + 2 Al → 1 2 A l 3 M g 2 + 1 2 Al + H 2 is endothermic in total and starts at about 240 °C, although the formation reaction of Al3Mg2 (3Al + 2Mg → Al3Mg2) is exothermic.

Thermodynamics and kinetics of the reversible dissociation of metal-doped NaAlH 4 as a hydrogen (or heat) storage system have been investigated in some detail. The experimentally determined enthalpies for the first (3.7 wt% of H) and the second dissociation step of Ti-doped NaAlH 4 (3.0 wt% H) of 37 and 47 kJ/mol are in accordance with low and medium temperature reversible metal hydride systems, respectively. Through variation of NaAlH 4 particle sizes, catalysts (dopants) and doping procedures, kinetics as well as the cyclization stability within cycle tests have been substantially improved with respect to the previous status [B. Bogdanovic, M. Schwickardi, J. Alloys Comp. 253–254 (1997) 1]. In particular, using combinations of Ti and Fe compounds as dopants, a cooperative (synergistic) catalytic effect of the metals Ti and Fe in enhancing rates of both de- and rehydrogenation of Ti/Fe-doped NaAlH 4 within cycle tests, reaching a constant storage capacity of ∼4 wt% H 2 , has been demonstrated. By means of 57 Fe Mossbauer spectroscopy of the Ti/Fe-doped NaAlH 4 before and throughout a cycle test, it has been ascertained that (1) during the doping procedure, nanosize metallic Fe particles are formed from the doping agent Fe(OEt) 2 and (2) already after the first dehydrogenation, the nanosize Fe particles with NaAlH 4 present are probably transformed into an Fe–Al-alloy which throughout the cycle test remains practically unchanged.

The catalytic reactions between monosilane or disilane and 1-hexene or 1,5-hexadiene were studied. The reactions with monosilane gave the silylated compounds in the presence of a platinum complex, LiAlH 4 and a radical initiator. In the reactions with disilane, the same silylated compounds were produced in the presence of a platinum complex as in the reaction with monosilane, and disilanylated compounds were obtained using LiAlH 4 and a radical initiator. The reaction mechanisms are discussed.

New reversible hydrogen storage systems are proposed, based on catalyzed reactions (Eqs. 4–6). The catalytic acceleration of the reactions in both directions is achieved by doping alkali metal aluminium hydrides with a few mol% of selected Ti compounds. The PCI diagrams of the Ti catalyzed systems show an absence of hysteresis and nearly horizontal pressure plateaus. The PCI of the NaAlH4 system reveals two temperature-dependent pressure plateaus, corresponding to the two-step reversible dissociation of NaAlH4. The PCI of the Na3AlH6 system shows only one pressure plateau; the latter can be lowered by partial substitution of Na by Li. In cyclic tests, reversible H2 capacities of 4.2–3.1 and 2.7–2.1 wt% H have been achieved.

Sodium alanate (NaAlH4) is a prototype system for storage of hydrogen in chemical form. However, a key experimental finding, that early transition metals (TMs) like Ti, Zr, and Sc are good catalysts for hydrogen release (and reuptake) whereas traditional hydrogenation catalysts like Pd and Pt are poor catalysts for NaAlH4, has so far received little attention. We performed density functional theory (DFT) calculations at the PW91 generalized gradient approximation level on Ti, Zr, Sc, Pd, and Pt interacting with the (001) surface of nanocrystalline NaAlH4, employing a cluster model of the complex metal hydride to study the initial mass transport in the dehydrogenation process. A key difference between Ti, Zr, and Sc on one hand and Pd and Pt on the other is that exchange of the early TM atoms with a surface Na ion, whereby Na is pushed on to the surface, is energetically preferred over surface absorption in an interstitial site, as found for Pd and Pt. These theoretical findings are consistent with a crucial feature of the TM catalyst being that it can be transported with the reaction boundary as it moves into the bulk, enabling the starting material to react away while the catalyst eats its way into the bulk and affecting a phase separation between a Na-rich and an Al-rich phase. Additional periodic DFT/PW91 calculations in which NaAlH 4 is modeled as a slab to model dehydrogenation of larger NaAlH 4 particles and which only consider adsorption and absorption of Ti suggest that Ti prefers to absorb interstitially but with only a small energy preference over a geometry in which Ti has exchanged with Na. Additional nudged elastic band calculations based on periodic DFT show only a small barrier (0.02 eV) for exchange of Ti with a surface Na atom. The mechanism inferred from the cluster calculations is therefore consistent with the slab calculations and may well be important. © 2012 American Chemical Society., The work presented here has been supported by a grant from the Dutch research council NWO under the ACTS Hydrogen programme and by a grant of computer time by the Dutch National Computing facilities Foundation (NCF).

Two materials currently of interest for onboard lightweight hydrogen storage applications are sodium aluminum hydride (NaAlH4), a complex metal hydride, and carbon aerogels (CAs), a light porous material connected by several spherical nanoparticles. The objectives of the present work have been to investigate the synthesis, characterization, and hydrogenation behavior of Pd-, Ti- or Fe-doped CAs, NaAlH4, and MgH2nanocomposites. The diameters of Pd nanoparticles onto CA’s surface and BET surface area of CAs were 3–10 nm and 700–900 m2g−1, respectively. The H2storage capacity of metal hydrides has been studied using high-pressure TGA microbalance and they were 4.0, 2.7, 2.1, and 1.2 wt% for MgH2-FeTi-CAs, MgH2-FeTi, CAs-Pd, and 8 mol% Ti-doped NaAlH4, respectively, at room temperature. Carbon aerogels with higher surface area and mesoporous structures facilitated hydrogen diffusion and adsorption, which accounted for its extraordinary hydrogen storage phenomenon. The hydrogen adsorption abilities of CAs notably increased after inclusion of metal hydrides by the “hydrogen spillover” mechanisms.

Safe and efficient hydrogen storage is a significant challenge inhibiting the use of hydrogen as a primary energy carrier. Although energy storage performance properties are critical to the success of solid-state hydrogen storage systems, operator and user safety is of highest importance when designing and implementing consumer products. As researchers are now integrating high energy density solid materials into hydrogen storage systems, quantification of the hazards associated with the operation and handling of these materials becomes imperative. The experimental effort presented in this paper focuses on identifying the hazards associated with producing, storing, and handling sodium alanates, and thus allowing for the development and implementation of hazard mitigation procedures. The chemical changes of sodium alanates associated with exposure to oxygen and water vapor have been characterized by thermal decomposition analysis using simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS) and X-ray diffraction methods. Partial oxidation of sodium alanates, an alkali metal complex hydride, results in destabilization of the remaining hydrogen-containing material. At temperatures below 70 C, reaction of sodium alanate with water generates potentially combustible mixtures of H{sub 2} and O{sub 2}. In addition to identifying the reaction hazards associated with the oxidation of alkali-metal containing complex hydrides, potential treatment methodsmore » are identified that chemically stabilize the oxidized material and reduce the hazard associated with handling the contaminated metal hydrides.« less

Presence of ∼2-4 % Ti is critical for reversible hydrogenation/rehydrogenation in NaAlH4. We have investigated the probable catalytic role of Ti in this complex multi-step process. The present part of our study concentrates on the rehydrogenation reaction, i.e., the reverse reaction that forms NaAlH4 from its constituent binary hydrides. First principles calculations using density functional theory (DFT) show that a particular arrangement of Ti atoms on the surface of Al metal promotes the chemisorption of molecular hydrogen. We also present comparisons with existing experimental data (EXAFS and TEM) to support the existence of such an arrangement on the surface.

Complex metal hydrides of general formula, ABH4 (A = alkali metals, B = third group elements such as B, Al, Ga) are potential candidates as hydrogen storage media for transportation. Thermal decomposition of complex hydrides generates hydrogen at elevated temperatures. The by -products of the dehydrogenation process can be regenerated using gaseous hydrogen at suitable temperature and pressure. The initial steps of thermal decomposition of NaAlH4 may be more complicated from the decomposition pathway reported in the literature. Close examination using thermal analysis by TGA, DSC and XRD measurements over the temperature range 30–500°C showed that the initial evolution of hydrogen occurred at a slow rate at ∼80°C, prior to fast decomposition at 190°C and at 260°C. Four regions of weight loss and five major endothermic peaks were measured during the thermal analysis. The effect of heating rate on the thermal analysis response showed that a high resolution of the thermal processes could be achieved at higher heating rates. Thermodynamic data was obtained for the various steps in the decomposition process including the formation of intermediate phases Na 3AlH6, and NaH. We also found that the decomposition of NaH is highly pressure dependent probably due to the high compressibility of the diffuse H− anion. The crystal-chemistry of NaAlH4 during decomposition has been established using X-ray diffraction analysis.

The authors studied BaReH{sub 9} because of interest with respect to hydrogen storage because it has the highest known H/M ratio. The structure was characterized by infrared spectroscopy, NMR, and X-ray diffraction.

The hydride-induced carbonylation of organoboranes proceeds via alkali metal trialkylborohydride intermediates regardless of the complex metal hydride used. Apparent variations in alkyl group migratory aptitudes observed previously, which were attributed to differences in complex hydride steric requirements, evidently arise instead from selective formation of products that are difficult to oxidize.

The reduction of cis-stilbene episulfone (I) by such a complex metal hydride as lithium borohydride, sodium borohydride, or lithium aluminum hydride gave dibenzylsulfone (V) selectively as a result of the unusual cleavage of the carbon-carbon single bond. Comparing the difference between lithium aluminum hydride and lithium borohydride or sodium borohydride in various solvent systems, we suggest that particular solvent effects are operating in the present reaction. The reduction of I with lithium aluminum hydride resulted in a low yield (0–10%) of V, while the reduction with lithium borohydride afforded V in a 45% yield. The reduction of styrene episulfone (II) gave no carbon-carbon cleavage product, but it did give a carbon-sulfur fission product, yielding sulfinic acid salt (VIII). The formation of VIII is interesting compared with the reduction of styrene oxide by lithium aluminum hydride. Tetraphenylethylene episulfone (III) produced a carbon-carbon cleavage product, 1, 1, 3, 3-tetraphenyl dimethyl su...

Reaction of LiCuMe2 at low temperature with LiAlH4 in diethyl ether results in the formation of a highly pyrophoric solid whose analysis is consistent with the formula LiCuH2, and which is stable as an etherate.

Reaction of LiCuMe2 at low temperature with LiAlH4 in diethyl ether results in the formation of a highly pyrophoric solid whose analysis is consistent with the formula LiCuH2, and which is stable as an etherate.