- Ca-Na-N-H system for reversible hydrogen storage
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Ca-Na-N-H system was introduced and evaluated in this paper for reversible hydrogen storage. Similar to other amide-hydride systems already reported, interaction between Ca(NH2)2-NaH (1/1) was observed in the temperature range of 120
- Xiong, Zhitao,Wu, Guotao,Hu, Jianjiang,Chen, Ping
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- Comparative study of dehydrogenation of sodium aluminum hydride wet-doped with ScCl3, TiCl3, VCl3, and MnCl2
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A comparative study of the dehydrogenation of pure, ScCl3-, TiCl3-, VCl3-, and MnCl2-doped sodium alanate is reported. The samples wet-doped with transition metal halides exhibit significant lowering of both fir
- Naik, Mehraj-ud-din,Rather, Sami-ullah,Zacharia, Renju,So, Chang Su,Hwang, Sang Woon,Kim, Ae Rhan,Nahm, Kee Suk
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- Structural determination of NaAl2Ga2 intermetallic compound having the ThCr2Si2 type structure
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NaAl2Ga2 intermetallic compound has been synthesized by direct combination of the elements in the atomic ratio Na:Ga:Al = 1:2:2. Guinier-H?gg X-ray and neutron powder diffraction determined a ThCr2Si2 type struc
- Kadir,Noréus
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- A new synthetic route to Mg2Na2NiH6 where a [NiH4] complex is for the first time stabilized by alkali metal counterions
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Mg2Na2NiH6 was synthesized by reacting NaH and Mg2NiH4 at 310°C under hydrogen pressure. The novel structure type was refined from neutron-diffraction data in the orthorhombic space group Pnma (No. 62), with unit cell dimensions of a = 11.428(2), b = 8.442(2), and c = 5.4165(9) A and a unit cell volume = 523 A3 (Z = 4). The structure can be described by (Mg 2H2)2+ layers intersected by (Na 2NiH4)2- layers. The [NiH4] 4- complex is approximately tetrahedral, indicating formal zerovalent nickel. This is the first example of a solid-state hydride where a [NiH 4]4- complex is directly stabilized by alkali metal ions instead of the more polarizing Mg2+ ions. A rather long nickel-hydrogen bond distance of 1.65 A indicates a weaker Ni-H bond as a result of the weaker support from the less polarizing alkali metal counterions.
- Kadir, Karim,Noreus, Dag
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- Na3RhH6, Na3IrH6 and Li3IrH6 - new complex hydrides with isolated [RhH6]3-- and [IrH6 ]3--octahedra
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The ternary alkali metal rhodium and iridium hydrides were synthesized by the reaction of alkali metal hydride with transition metal powder in a pure hydrogen atmosphere. The crystal structures were determined by X-ray investigations on powdered samples and elastic neutron diffraction experiments on the deuterated compounds. The isotypic atomic arrangements (space group Pnma) contain isolated [RhH6]3-- and [IrH6]3--octahedra which are separated by the alkali metal ions.
- Bronger, W.,Gehlen, M.,Auffermann, G.
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- The millimeter wave spectra of NaH and NaD
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Utilizing a glow discharge absorption cell, we have detected the v = 0, 1, 2, and 3, J = 0 -> 1 transitions of NaH and the v = 0, 1, 2, and 3, J = 1 -> 2 and v = 0, J = 2 -> 3 transitions of NaD in the millimeter and submillimeter regions of the spectrum.The derived Dunham constants (MHz) are .A significant breakdown of the Born-Oppenheimer approximation has been observed.
- Sastry, K. V. L. N.,Herbst, Eric,Lucia, Frank C. De
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- Preparation of alkali metal hydrides by mechanical alloying
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Rubidium and cesium hydrides are not commercialized and we have set up, a few years ago, a method of synthesis at the laboratory scale. It is based on the reaction of alkali metal with hydrogen obtained by thermal decomposition of uranium hydride UH3 at a temperature of 450°C, which gives a pressure of hydrogen close to 3 bars. This synthesis leads to a very pure alkali metal hydride MH, but the rate of the reaction remains quite small: a few hundreds of milligrams in 24 h. A new method, based on mechanical alloying, consists in milling the alkali metal, at room temperature, under a pressure of hydrogen close to 5 bars. The reaction proceeds in 16 h and gives 3-15 g of very pure MH (from sodium to cesium, respectively) at once.
- Elansari,Antoine,Janot,Gachon,Kuntz,Guérard
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- PdH2.
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The ternary hydride Na//2PdH//2 was synthesized by the reaction of sodium hydride with palladium in a hydrogen atmosphere at 370 degree C. The structure was derived from X-ray investigations on powdered samples and on a single crystal as well as from neutron diffraction experiments on the deuterated compound. Na//2PdH//2 crystallizes in the tetragonal space group I4/mmm and is isotypic with Na//2HgO//2. The atomic arrangement is characterized by a novel linear left bracket PdH//2 right bracket complex.
- Noreus,Tornroos,Borje,Szabo
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- Laser-production of NaH crystalline particles
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Production of NaH crystalline particles of μm size is observed in sodium vapor mixed with ca. 10 Torr H2.The particles are produced when Na2 is excited to the B 1Πu state by a cw Ar+ laser, and also when Na is excited by a cw dye laser tuned to the D1 or D2 line.
- Yabuzaki, T.,Sato, T.,Ogawa, T.
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- Mechanochemically driven nonequilibrium processes in MNH 2-CaH2 systems (M = Li or Na)
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Mechanochemical transformations of lithium and sodium amides with calcium hydride have been investigated using gas volumetric analysis, X-ray powder diffraction, and residual gas analysis. The overall mechanochemical transformations are equimolar, and the
- Dolotko, Oleksandr,Zhang, Haiqiao,Li, Sa,Jena, Puru,Pecharsky, Vitalij
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- Improved molecular constants for the X1∑+ and a1∑+ states of NaH
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Improved molecular constants for the X1∑+ and A1∑+ states of the NaH molecule are presented. NaH molecules are produced by reactive scattering of H and Na2 in a crossed beam experiment. High vibrational levels (6 ≤ υ″ ≤ 9) of the NaH molecules are predominantly populated. Their excitation spectrum in the range 630 nm ≤ λ ≤ 670 nm has been measured using a new variant of Doppler spectroscopy. The transition frequencies involving the vibrational levels 2 ≤ υ′ ≤ 8 in the A1∑+ and 6 ≤ υ″ ≤ 9 in the X1∑+ state have been determined with an accuracy of better than 0.01 cm-1. Using also previously published data a new set of molecular constants for the X2∑+ and A1∑+ state is derived. In particular, the vibrational dependence of the rotational constants B, D and H as well as some of υ″-υ′ band origins for 0 ≤ υ″ ≤ 9 and 0 ≤ υ′ ≤ 25 is determined. The transition frequencies measured here or published previously are reproduced by these new coefficients with an accuracy of ≤ 0.1 cm-1 [rms value] with a maximum deviation of 0.4 cm-1. New RKR potential energy curves have been calculated up to the turning points of the levels υ″ = 9 in the X-1∑1+ state and υ′ = 25 in the A1∑+ state.
- Pesl,Lutz,Bergmann
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- New Dunham coefficients of the A1Σ+-State of NaH and NaD
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We determined new Dunham coefficients of the A1Σ+-State of NaH and NaD from degenerate Four-Wave-Mixing (DFWM) spectra in the near UV and in the blue spectral region. In the case of NaH we combined these data with results of Rafi et al. and Orth et al.. The new set of coefficients describes the vibrational dependence of the rotational constants B and D and of the band origins from v′ = 0 up to 25. The spectral positions of the lines in our DFWM-spectra can be reproduced by this coefficients with an accuracy better than 0.3 cm-1 for J-values ≤ 15 and 2 cm-1 for J-values ≤ 25. Especially for high J-values this is an improvement up to 30 times in comparison to Dunham coefficients recommended before. A RKR-potential of the A1Σ+-State was calculated with the new coefficients. The Dunham coefficients of NaD were obtained by scaling the NaH coefficients with the reduced masses of the molecules. A comparison of our results to molecular constants determined from the measured NaD-spectra shows good agreement. Springer-Verlag 1996.
- Lochbrunner,Motzkus,Pichler,Kompa,Hering
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- HIGHLY REACTIVE METAL HYDRIDES, PROCESS FOR THEIR PREPARATION AND USE
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The invention relates to powdery, highly reactive alkali and alkaline earth hydride compounds, and to mixtures with elements of the third main group of the periodic table of elements (PTE) and to the preparation thereof by reacting alkali or alkaline earth metals in the presence of finely dispersed metals or compounds of the third main group of the PTE, wherein the latter have one or more hydride ligands or said hydride ligands are converted in situ, under the prevailing reaction conditions, i.e., in the presence of hydrogen gas or another H source, into hydride species, and to the use thereof for the preparation of complex hydrides and organometallic compounds.
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Paragraph 0066-0071
(2018/06/29)
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- Preparations and de/re-hydrogenation properties of LixNa3-xAlH6 (x=0.9–1.3) non-stoichiometric compounds
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Mixed alkali alanates LixNa3-xAlH6 have been successfully synthesized by means of grinding mixtures of Li3AlH6 and Na3AlH6 in specific molar ratios. Non-stoichiometric Lix/
- Fan, Xiaolu,Zhang, Yao,Zhu, Yunfeng,Phillips, Cassandra,Guo, Xinli,Chen, Jian,Wang, Zengmei,Li, Liquan
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p. 648 - 654
(2017/09/28)
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- Structure, thermal analysis and dehydriding kinetic properties of Na1-xLixMgH3 hydrides
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NaMgH3 hydride with perovskite structure has been synthesized by high-energy ball milling, the maximum hydrogen-desorbed amount of which is 3.42 wt.% at 638 K. Two decomposition steps have been detected for perovskite-type NaMgH3 hydride, calculated values of activation energy for the two steps are 180.25 ± 8.25 kJ/mol and 156.23 ± 18.54 kJ/mol by Kissinger method. In comparison with NaMgH3 hydride, Li0.5Na0.5MgH3 hydride has better dehydriding kinetic properties and higher hydrogen-desorbed amount (4.11 wt.%) due to partial replacement of Na by Li. LiMgH3 hydride with perovskite structure cannot be synthesized by milling of the mixture of LiH and MgH2 hydrides. However, the maximum hydrogen-desorbed amount of this milled mixture is 5.54 wt.% at 638 K, this may suggest that LiH is a good catalyst for dehydrogenation of MgH2, but further research is needed.
- Wang, Zhong-Min,Li, Jia-Jun,Tao, Song,Deng, Jian-Qiu,Zhou, Huaiying,Yao, Qingrong
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p. 402 - 406
(2015/12/08)
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- Thermodynamic destabilisation of MgH2 and NaMgH3 using Group IV elements Si, Ge or Sn
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The addition of Group IV elements of Si, Ge or Sn to Mg-based hydrides has led to the successful destabilisation of MgH2 or NaMgH3, resulting in hydrogen release at lower temperatures. This is the first time a direct comparison has b
- Chaudhary, Anna-Lisa,Paskevicius, Mark,Sheppard, Drew A.,Buckley, Craig E.
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p. 109 - 116
(2015/02/19)
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- In situ synchrotron X-ray diffraction study on the improved dehydrogenation performance of NaAlH4-Mg(AlH4)2 mixture
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The dehydrogenation performance and mechanism of the synthesized NaAlH 4-Mg(AlH4)2 powders were investigated by performing thermogravimetric analysis and in situ synchrotron X-ray diffraction analysis. NaAlH4 not only facilitates the first step dehydrogenation of Mg(AlH4)2 in lowering its initial dehydrogenation temperature but also increases the total amount of H2 released. Besides, MgH2 and/or Al phases, the products of the first step dehydrogenation reaction, play a catalytic role in lowering the initial dehydrogenation temperature of NaAlH4. The synthesized NaAlH4-Mg(AlH4) 2 mixture has an initial dehydrogenation temperature as low as 120 °C, and is able to release 5.35 wt% H2 below 350 °C. The self-catalytic dehydrogenation behavior of the NaAlH4-Mg(AlH4) 2 mixture was elaborated in this work with the aid of in situ synchrotron XRD.
- Yang, Cheng-Hsien,Chen, Tzu-Teng,Tsai, Wen-Ta,Liu, Bernard Haochih
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- PbTe nanostructures: Microwave-assisted synthesis by using lead Schiff-base precursor, characterization and formation mechanism
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Pure cubic phase lead telluride (PbTe) nanostructures have been produced by using a Schiff-base complex as a precursor in the presence of microwave irradiation. The Schiff base used as ligand was derived from salicylaldehyde and ethylenediamine. The Schiff-base complex was marked as [Pb(salen)]. In addition, the effect of the irradiation time and the type of reducing agent on the morphology and purity of the final products was investigated. The as-synthesized PbTe nanostructures were characterized extensively by techniques like X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The microwave formation mechanism of the PbTe nanostructures was studied by XRD patterns of the products. Although it was found that both ionic and atomic mechanisms could take place for the preparation of PbTe, the main steps were according to the atomic reaction process, which could occur between elemental Pb and Te.
- Ahmadian-Fard-Fini, Shahla,Salavati-Niasari, Masoud,Monfared, Azam,Mohandes, Fatemeh
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p. 778 - 788
(2013/09/02)
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- Mechanochemical transformations in NaNH2-MgH2 mixtures
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Mechanochemical transformations occurring during ball milling of sodium amide (NaNH2) with magnesium hydride (MgH2) taken in 2:3 and 2:1 molar ratios have been investigated using X-ray powder diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) techniques. For the 2NaNH2-3MgH2 system the mechanochemical reaction proceeds via the formation of MgNH as an intermediate, whereas magnesium nitride (Mg 3N2), sodium hydride (NaH) and hydrogen (～5 wt%) form as the final products. The overall solid state reaction for this system is 2NaNH2 + 3MgH2 → Mg3N2 + 2NaH + 4H2. However, the mechanochemical transformation of the 2NaNH 2-MgH2 system proceeds through the reaction: 2NaNH 2 + MgH2 → Mg(NH2)2 + 2NaH, without any hydrogen release. Comparison of the mechanochemical transformations with the previously studied thermochemical transformations reveals that the two approaches lead to the same final products via different reaction pathways.
- Singh, Niraj K.,Kobayashi, Takeshi,Dolotko, Oleksandr,Wiench, Jerzy W.,Pruski, Marek,Pecharsky, Vitalij K.
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p. 324 - 327
(2012/03/22)
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- Reversible hydrogen storage in Ti-Zr-codoped NaAlH4 under realistic operation conditions
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Ti-Zr-codoped NaAlH4 exhibits improved hydrogen desorption and reabsorption properties compared with sole Ti- or Zr-doped alanate. This contribution aims on reversible hydrogen storage in such material under realistic operation conditions. Results on isothermal dehydrogenation-rehydrogenation cycles at 125 °C and desorption at 4 bar hydrogen back-pressure are presented, proving NaAlH4 to be a suitable hydrogen material in combination with proton exchange membrane fuel cells.
- Schmidt, Thomas,R?ntzsch, Lars
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- Investigations on the solid state interaction between LiAlH4 and NaNH2
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In this paper, two LiAlH4NaNH2 samples with LiAlH4 to NaNH2 molar ratio of 1/2 and 2/1 were investigated, respectively. It was observed that both samples evolved 2 equiv H2 in the ball milling process
- Chua, Yong Shen,Wu, Guotao,Xiong, Zhitao,Chen, Ping
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p. 2040 - 2044
(2011/01/06)
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- High temperature-pressure processing of mixed alanate compounds
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Mixtures of light-weight hydrides and elements were investigated to increase the understanding of the chemical reactions that take place between various materials. This report details investigations we have made into mixtures that include NaAlH4, LiAlH4, MgH2, Mg2NiH4, alkali(ne) hydrides, and early third row transition metals (V, Cr, Mn). Experimental parameters such as stoichiometry, heat from ball milling versus hand milling, and varying the temperature of high pressure molten state processing were studied to examine the effects of these parameters on the reactions of the complex metal hydrides.
- Berseth, Polly A.,Pittman, Jennifer,Shanahan, Kirk,Stowe, Ashley C.,Anton, Donald,Zidan, Ragaiy
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p. 2141 - 2145
(2009/02/05)
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- Rotational and vibrational state distributions of NaH in the reactions of Na (4 S2,3 D2, and 6 S2) with H2: Insertion versus harpoon-type mechanisms
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By using a pump-probe technique, the nascent rotational and vibrational state distributions of NaH are obtained in the Na (4 S2,3 D2, and 6 S2) plus H2 reactions. The rotational distributions for the Na (4 S2,3 D2) reactions yield a bimodal feature with a major component peaking at J=20-22, similar to that obtained previously in the 4 P2 reaction, whereas the Na (6 S2) reaction gives rise to a distinct distribution with a much lower rotational temperature. The vibrational populations (v=0-4) for these 4 S2, 3 D2, and 6 S2 reactions are characterized by corresponding temperatures of 1692±120, 819±35, and 5329±350 K. Due to a significant contribution of configurational mixing between different states with the same symmetry, the collision species initiated from the 4 S2 and 3 D2 states are anticipated to track along the entrance surface in a near C2v symmetry, then undergo nonadiabatic transition to the inner limb of the reactive 2 A′ surface. In contrast, the reaction pathway for the Na (6 S2) state with a significantly reduced ionization energy is anticipated to follow a harpoon-type mechanism via a (near) collinear configuration. The increased atomic size of Na may hinder the insertion approach.
- Chang, Yuan-Pin,Hsiao, Ming-Kai,Liu, Dean-Kuo,Lin, King-Chuen
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- Synthesis and characterization of amide-borohydrides: New complex light hydrides for potential hydrogen storage
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The reactions xLiNH2 + (1 - x)LiBH4 and xNaNH2 + (1 - x)NaBH4 have been investigated and new phases identified. The lithium amide-borohydride system is dominated by a body centred cubic compound of formula Li4BH4(NH2)3. In the sodium system, a new hydride of approximate composition Na2BH4NH2 has been identified with a primitive cubic structure and lattice parameter a ≈ 4.7 ?. The desorption of gases from the two amide-borohydrides on heating followed a similar pattern with the relative proportions of H2 and NH3 released depending critically on the experimental set-up: in the IGA, ammonia release occurred in two steps - beginning at 60 and 260 °C for Li4BH4(NH2)3 - the second of which was accompanied by hydrogen release; in the TPD system the main desorption product was hydrogen-again at 260 °C for Li4BH4(NH2)3 accompanied by around 5% ammonia. We hypothesize that the BH4- anion can play a similar role to LiH in the LiNH2 + LiH system, where ammonia release is suppressed in favour of hydrogen. The reaction xLiNH2 + (1 - x)LiAlH4 did not result in the production of any new phases but TPD experiments show that hydrogen is released from the mixture 2LiNH2 + LiAlH4, over a wide temperature range. We conclude that mixed complex hydrides may provide a means of tuning the dehydrogenation and rehydrogenation reactions to make viable storage systems.
- Chater, Philip A.,Anderson, Paul A.,Prendergast, James W.,Walton, Allan,Mann, Vicky S.J.,Book, David,David, William I.F.,Johnson, Simon R.,Edwards, Peter P.
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p. 350 - 354
(2008/10/09)
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- How carbon affects hydrogen desorption in NaAlH4 and Ti-doped NaAlH4
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The hydrogen storage properties of doped and undoped NaAlH4 samples are studied after mixing them with different percentages of high surface carbon. Manually mixed samples are compared with ball milled ones; it was found that manual mixing was a simple and effective way to dope NaAlH4. A morphological and micro-structural analysis has been carried out in order to understand the effect of carbon. Carbon added samples show a marked enhancement of hydrogen desorption rate. The desorption temperature and the total hydrogen content remain almost unchanged for undoped sample. The desorption temperatures of Ti-doped samples increase with carbon content.
- Cento,Gislon,Bilgili,Masci,Zheng,Prosini
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p. 360 - 366
(2008/10/09)
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- Integrated experimental-theoretical investigation of the Na-Li-Al-H system
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First-principles modeling, experimental, and thermodynamic methodologies were integrated to facilitate a fundamentally guided investigation of quaternary complex hydride compounds within the bialkali Na-Li-Al-H hydrogen storage system. The integrated approach has broad utility for the discovery, understanding, and optimization of solid-state chemical systems. Density functional theory ground-state minimizations, low-temperature powder neutron diffraction, and low-temperature synchrotron X-ray diffraction were coupled to refine the crystallographic structures for various low-temperature distorted Na2LiAlH6 allotropes. Direct method lattice dynamics were used to identify a stable Na2-LiAlH6 allotrope for thermodynamic property predictions. The results were interpreted to propose transformation pathways between this allotrope and the less stable cubic allotrope observed at room temperature. The calculated bialkali dissociation pressure relationships were compared with those determined from pressure-composition-isotherm experiments to validate the predicted thermodynamic properties. These predictions enabled computational thermodynamic modeling of Na2LiAlH6 and competing lower order phases within the Na-Li-Al-H system over a wide of temperature and pressure conditions. The predictions were substantiated by experimental observations of varying Na2LiAlH6 dehydrogenation behavior with temperature. The modeling was used to identify the most favorable reaction pathways and equilibrium products for H discharge/recharge in the Na-Li-Al-H system, and to design conditions that maximize the theoretical hydrogen reversibility within the Na-Li-Al-H system.
- Opalka, Susanne M.,Lovvik, Ole M.,Brinks, Hendrik W.,Saxe, Paul W.,Hauback, Bjoern C.
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p. 1401 - 1409
(2008/10/09)
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- Design, fabrication and testing of NaAlH4 based hydrogen storage systems
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To complement the vigorous search for novel hydrogen storage materials, efforts focused on system implementation of candidate compounds are important parallel activities to identify new or reprioritized system challenges and assess overall performance. The current paper will discuss the design, fabrication and testing of on-board rechargeable storage systems based on the complex hydride NaAlH4. Emphasis is placed on the system elements affected by the different material characteristics compared with conventional metal hydrides such as LaNi5. Design aspects include reaction kinetics modeling, finite element analysis and heat exchanger optimization. Materials related fabrication challenges are discussed associated with catalysis processing and powder densification. Testing facilities and techniques to evaluate a full-scale vessel containing nearly 20 kg of NaAlH4 also are covered.
- Mosher,Arsenault,Tang,Anton
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p. 707 - 712
(2008/10/09)
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- Synthesis of NaAlH4-based hydrogen storage material using milling under low pressure hydrogen atmosphere
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The present study highlights the advantages of milling NaH/Al under moderate hydrogen pressure as a favourable production step for NaAlH4-based hydrogen storage materials. Firstly, it is demonstrated that NaAlH4 can be obtained by applying a moderate hydrogen pressure (6-12 bar) during milling of NaH and Al with and without the presence of an inexpensive catalyst (TiCl4). The yield of NaAlH4 depends critically on process parameters, such as hydrogen pressure and milling time. A fully converted product is capable of reversible hydrogen storage without any activation procedure. Under optimized conditions, a capacity of 4.2 wt.% was achieved and kinetics in the first desorption are comparable to NaAlH4 doped with TiCl3. Secondly, the synthesis has been optimized towards shorter milling times. By applying a few absorption/desorption cycles to material that was partially converted during milling, almost full reversible storage capacity can be reached. In addition, kinetics is extremely enhanced. For example, such material exhibits an optimum capacity already after two sorption cycles at 100 bar and 125 °C and allows to absorb 80% of the reversible hydrogen content within a few minutes.
- Eigen, Nico,Kunowsky, Mirko,Klassen, Thomas,Bormann, Rüdiger
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p. 350 - 355
(2008/10/09)
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- PHYSIOCHEMICAL PATHWAY TO REVERSIBLE HYDROGEN STORAGE
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In one embodiment of the present disclosure, a process for cyclic dehydrogenation and rehydrogenation of hydrogen storage materials is provided. The process includes liberating hydrogen from a hydrogen storage material comprising hydrogen atoms chemically bonded to one or more elements to form a dehydrogenated material and contacting the dehydrogenated material with a solvent in the presence of hydrogen gas such that the solvent forms a reversible complex with rehydrogenated product of the dehydrogenated material wherein the dehydrogenated material is rehydrogenated to form a solid material containing hydrogen atoms chemically bonded to one or more elements.
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Page/Page column 12-17
(2008/06/13)
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- Catalytic effect of Al3Ti on the reversible dehydrogenation of NaAlH4
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Al3Ti was directly utilized as catalyst precursor to examine its catalytic activity in the reversible dehydrogenation of NaAlH4. It was observed that Al3Ti possessed considerable catalytic effect on the de-/hydriding react
- Kang,Wang,Song,Yao,Lu,Cheng
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p. 365 - 369
(2008/10/09)
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- Comparative studies of the decomposition of alanates followed by in situ XRD and DSC methods
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The decomposition of various alkali and alkaline earth complex alanates and the formation of intermediate compounds was studied by in situ X-ray high-temperature diffraction and differential scanning calorimetry (DSC) experiments. Differences of the reaction pathways during thermolysis for the alkali and the alkaline earth aluminum hydrides were determined. During the thermolysis of Mg(AlH4)2 and Ca(AlH4)2, the appearance of metal hydrides in combination with alloys was observed, whereas for the alkali alanates LiAlH4 and KAlH4, intermediate aluminum hydrides but no alloys are formed. For the alkali salt-containing KAlH4 systems a strong influence due to the presence of salts on the decomposition temperatures are observed. In addition, the decomposition temperatures are also significantly influenced by the type of salt present. For the first time, the decomposition of the LiMg(AlH4)3 and Na2LiAlH6 systems was studied by in situ methods.
- Mamatha,Weidenthaler,Pommerin,Felderhoff,Schüth
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p. 303 - 314
(2008/10/09)
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- Evidence for the existence of β-Na3AlH6: Monitoring the phase transformation from α-Na3AlH6 by in situ methods
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The phase transformation of α-Na3AlH6 to β-Na3AlH6 was characterized by in situ DSC and high-temperature X-ray diffraction methods. The detection of the phase transformation from the α- to the β-polymorph requires rapid heating rates and a very fast data acquisition. The influence of the preparation method on the stability of both polymorphic forms most probably originates from different particle sizes of the parent samples.
- Weidenthaler,Pommerin,Felderhoff,Schmidt,Bogdanovi?,Schüth
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p. 228 - 234
(2008/10/09)
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- Structures and thermodynamics of the mixed alkali alanates
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The thermodynamics and structural properties of the hexahydride alanates(M2 M′ Al H6) with the elpasolite structure have been investigate d. A series of mixed alkali alanates (Na2 LiAl H6, K2 LiAl H6, and K2 NaAl H6) were synthesized and found to reversibly absorb and desorb hydrogen without the need for a catalyst. Pressure-composition isotherms were measured to investigate the thermodynamics of the absorption and desorption reactions with hydrogen. Isotherms for catalyzed (4 molpercent Ti Cl3) and uncatalyzed Na2 LiAl H6 exhibited an increase in kinetics, but nochange in the bulk thermodynamics with the addition of a dopant. A stru ctural analysis using synchrotron x-ray diffraction showed that these compounds favor the Fm 3 m space group with the smaller ion (M′) occupying an octahedral site. These results demonstrate that appropriate cation substitutions can be used to stabilize or destabilize the material and may provide an avenue to improving the unfavorable thermodynamics ofa number of materials with promising gravimetric hydrogen densities.
- Graetz, J.,Lee, Y.,Reilly, J. J.,Park, S.,Vogt, T.
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- Direct formation of Na3 Al H6 by mechanical milling NaHAl with Ti F3
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Na3 Al H6 can be directly formed by mechanical milling NaHAl with Ti F3 under hydrogen atmosphere. The hydrogenation fraction of NaH increases with increasing the milling time, and reaches up to 0.61 after 20 h milling. Thus-formed Na3 Al H6 exhibits unexpected polymorphic transformation and decomposition behaviors. This, together with the unusual hydrogen storage performance of the mechanically prepared materials, provides us a suggestive perspective to probe the favorable modification of the thermodynamics of Na3 Al H6 and nature of active Ti-species in Ti-doped NaAl H4.
- Wang,Kang,Cheng
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- Effects of catalysts on the dehydriding of alanates monitored by proton NMR
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In situ studies of the transition from NaAlH4 to Na 3AlH6 are performed by proton NMR for samples doped with TiCl3 and with Ti13-nanoclusters. The local hydrogen dynamics in the different compounds is studied by the nuclear spin-lattice relaxation. For the Ti-doped NaAlH4 samples a double-exponential recovery of the nuclear magnetization is observed, indicating two fractions of hydrogen with different mobilities. In Na3AlH6 a transition from hindered rotation of the AlH6 groups to full isotropic reorientation of these groups is observed in the temperature range 200-260 K.
- Majer,Stanik,Banuet, L.E. Valiente,Grinberg,Kircher,Fichtner
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p. 738 - 742
(2008/10/09)
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- Motion of point defects and monitoring of chemical reactions in sodium aluminium hydride
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Anelastic spectroscopy experiments (elastic modulus and energy dissipation) were carried out for the first time with Ti-doped and undoped NaAlH 4. We found that the various decomposition reactions taking place in the alanates are most sensitively monitored by the dynamic Young modulus variations. We also found that, during one of the decomposition reactions occurring at higher temperature, a new point-defect complex is formed, very likely involving hydrogen, which has fast dynamics and gives rise to a thermally activated relaxation process at 70 K in the kHz range, with an activation energy of 0.126 eV. The occurrence of this process suggests that any model concerned with the decomposition mechanism should take into account the hydrogen mobility in the crystal lattice.
- Palumbo,Cantelli,Paolone,Jensen,Srinivasan
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p. 748 - 751
(2008/10/09)
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- Method for preparing Ti-doped NaAlH4 using Ti powder: Observation of an unusual reversible dehydrogenation behavior
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Titanium powder can be directly used as dopant in the preparation of catalytically enhanced Ti-doped NaAlH4 upon mechanical milling under an atmosphere of hydrogen. The hydrogen storage performance of NaAlH4 that was doped through th
- Wang,Jensen
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- Long term cycling behavior of titanium doped NaAlH4 prepared through solvent mediated milling of NaH and Al with titanium dopant precursors
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A simple and an efficient synthesis route, solvent mediated milling of NaH and Al with 2mol% of the dopant precursor, Ti(OBu)4 followed by hydrogenation, has been developed and employed to synthesize Ti-doped NaAlH 4. The long-term hydrogenation and dehydrogenation, up to 100cycles were carried out systematically. Reversibility of about 3.4wt.% hydrogen release was obtained during the first dehydrogenation (160°C) run after the initial hydrogenation of Ti-doped (NaH+Al) at 150°C; ～11.4MPa H 2 for 12h. In the subsequent cycles, the storage capacity increased, reaching an optimum of 4.0wt.%. This capacity was retained for 40cycles with the dehydrogenation kinetic curves showing remarkable reproducibility. Comparison of the X-ray diffraction profiles of Ti-doped (NaH+Al) from initial and final stages of the cycling study reveals a growing resistance to the hydrogenation of Na3AlH6 to NaAlH4.
- Srinivasan, Sesha S.,Brinks, Hendrik W.,Hauback, Bjorn C.,Sun, Dalin,Jensen, Craig M.
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p. 283 - 289
(2008/10/09)
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- A kinetics model of hydrogen absorption and desorption in Ti-doped NaAlH4
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The kinetics of the hydrogen sorption of Ti-doped direct-synthesized NaAlH4 has been studied by measuring sorption rates at various temperatures and applied pressures. Formation and decomposition rate equations for NaAlH4 and Na3AlH6 are proposed, and pre-exponential factors and activation energies have been calculated for these reactions. These equations were used to calculate alanate decomposition curves. The results fit the experimental data very well. The predictive capabilities of this empirical approach provide a very useful modeling tool for optimizing the performance of the alanates over a range of hydrogen absorption temperatures and pressures. Under a typical hydrogen-loading condition (constant pressure), the optimum temperature for fast fill can be determined.
- Luo, Weifang,Gross, Karl J.
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p. 224 - 231
(2008/10/09)
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- Titanium-halide catalyst-precursors in sodium aluminum hydrides
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The kinetics of absorption and desorption of hydrogen from NaAlH4 have previously been shown to improve upon the addition of a catalyst-precursor such as TiCl3. In this paper we demonstrate that TiCl2, TiF3, and TiBr4 all effectively improve sorption kinetics. Arrhenius data indicate that the catalyst precursors behave in essentially the same manner. Evidently the valency of Ti in the catalyst-precursor is inconsequential to the role of Ti in altering the kinetic mechanism. The formation of TiAl3 on doping with TiCl3 has been observed. The presence of TiAl3 appears to contribute in part to the enhanced kinetics in these systems.
- Majzoub,Gross
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p. 363 - 367
(2008/10/08)
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- Rehydrogenation of dehydrogenated NaAlH4 at low temperature and pressure
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NaAlH4 is considered as a promising material for onboard storage of hydrogen. It is generally thought that high temperature (> 120?°C) and high pressure (> 15 MPa) are needed for rehydrogenation of dehydrogenated NaAlH4. However, for practical application, reducing the rehydrogenation temperature and pressure as much as possible would be highly desirable. We have found that the dehydrogenated NaAlH4, in either the undoped or titanium-doped state, can be rehydrogenated at temperatures of 25-120?°C and pressures of 2-12 MPa. This finding is confirmed by the subsequent dehydrogenation at 160?°C. In addition to that, after rehydrogenation under the same conditions, the amount of evolved hydrogen from the titanium-doped NaAlH4 was much higher than that of the undoped NaAlH4. This clearly indicates that the addition of titanium species enhances kinetically not only the dehydrogenation, but also the reverse process, the rehydrogenation reaction of the dehydrogenated NaAlH4.
- Sun, Dalin,Srinivasan, Sesha S.,Kiyobayashi, Tetsu,Kuriyama, Nobuhiro,Jensen, Craig M.
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p. 10176 - 10179
(2007/10/03)
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- Phase changes and hydrogen release during decomposition of sodium alanates
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We have constructed a hermetically sealed high-temperature cell for an X-ray diffractometer which allows in situ identification of structural phase changes during the several thermal decomposition stages of sodium alanate materials. Comparing X-ray data with thermogravimetry measurements allows identification of phase changes that correlate with hydrogen release. Measurements made while uniformly ramping up the temperature of purified NaAlH4 indicate that all of the hydrogen release corresponds to the formation of Al or the formation and decomposition of the Na3AlH6 phase. The melting of the NaAlH4 phase does not correlate well with sample mass loss in the purified material. For as-received technically pure NaAlH4, hydrogen release lags both Al formation and the formation and decomposition of the Na3AlH6 phase. In ball-milled as-received NaAlH4 (ball-milled with diamond powder to achieve ～2 μm crystallite size) there is good correspondence between hydrogen release and either Al formation or the formation and decomposition of the Na3AlH6 phase, except that hydrogen release lags the phase changes by a few degrees. For purified NaAlH4 ball-milled in either a steel or tungsten carbide jar, there is a much smaller lag in hydrogen release. For Ti doped material, the phase changes to NaH are completed at the lowest temperature observed in any of the samples, 215°C, but the TGA measurements are not reliable because of mass loss from decomposition of the catalyst butoxide anion. Among these samples, the delay in hydrogen release does not correlate well with crystallite size; indicating that the delay is not due to hydrogen diffusion, but the delay is definitely decreased by ball milling or adding catalyst material or even impurities.
- Balogh,Tibbetts,Pinkerton,Meisner,Olk
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p. 136 - 144
(2008/10/08)
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- Hydrogen reactivity of Li-containing hydrogen storage materials
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In hydrogen energy systems, the hydrogen storage alloy is expected as one of important materials for energy storage and transportation. However, they weigh so much for sufficient amounts of hydrogen, therefore, light hydrogen storage materials with much higher storage capacities are required. Alkaline complex hydrides are promising candidates as light hydrogen storage materials. In this study we tried to prepare a sample based on the Na-Li-Al system using the dry process of ball-milling as mechanical synthesis and investigate their hydrogen reaction characteristics. The same study for the sample with 10 wt.% La2O3 was also carried out and its hydrogen reactivity compared. The starting materials were LiAlH4, NaH. They were ball-milled with hexane dehydrated in 1 bar Ar. The hydrogen reactivity of the samples obtained were measured by the high-pressure Sievert's type apparatus. For the sample of NaH+LiH+Al, about 1.9～2.0 wt.% of hydrogen was absorbed reversibly within 150 min. The addition of 10 wt.% La2O3 improved both the ab/desorption reaction rate and cyclable hydrogen capacity.
- Genma,Uchida,Okada,Nishi
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p. 358 - 362
(2008/10/08)
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- Investigation of hydrogen discharging and recharging processes of Ti-doped NaAlH4 by X-ray diffraction analysis (XRD) and solid-state NMR spectroscopy
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The processes occurring in the course of two sequential hydrogen discharging and recharging cycles of Ti-doped sodium alanate were investigated in parallel using XRD analysis and solid-state NMR spectroscopy. Both methods demonstrate that in hydrogen storage cycles (Eq. (1)) the majority phases involved are NaAlH4, Na3AlH6, Al and NaH. Only traces of other, as yet unidentified phases are observed, one of which has been tentatively assigned to an Al-Ti alloy on the basis of XRD analysis. The unsatisfactory hydrogen storage capacities heretofore observed in cycle tests are shown to be due entirely to the reaction of Na3AlH6 with Al and hydrogen to NaAlH4 (Eq. (1), 2nd hydrogenation step) being incomplete. Using XRD and NMR methods it has been shown that a higher level of rehydrogenation can be achieved by adding an excess of Al powder.
- Bogdanovi?,Felderhoff,Germann,H?rtel,Pommerin,Schüth,Weidenthaler,Zibrowius
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p. 246 - 255
(2008/10/08)
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- Enhancing low pressure hydrogen storage in sodium alanates
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We investigated the dehydrogenation of NaAlH4 and the reversible low-pressure rehydrogenation from NaH to Na3AlH6. Highly purified NaAlH4 requires relatively high temperatures to decompose to NaH and long durations to rehydride to the Na3AlH6 phase in hydrogen gas. However, any degradation of the purity of this material, whether through ball milling with diamond powder or ball milling with diamond plus Al powders, mixing the purified material with Pt powder, or doping with a Ti organometallic compound, lowers the decomposition temperature and facilitates rehydriding the product NaH+Al to Na3AlH6. Diamond ball milling of NaAlH4 seems to be the best of these procedures; it substantially decreases the decomposition temperatures, with significant dehydrogenation starting at 180°C rather than 250°C for the purified material, and with formation of NaH substantially complete at 235°C rather than 290°C. Rather surprisingly, it also facilitates rehydrogenation from NaH+Al to Na3AlH6. Similarly, NaAlH4 doped with Ti according to the recipe of Bogdanovic? lowers the decomposition temperatures and improves the hydrogenation kinetics for the low pressure transition from NaH+Al to Na3AlH6. Pressure-composition isotherms show that the rehydrogenation of the resulting NaH+Al decomposition phases into the Na3AlH6 intermediate phase at pressures below 3.6 MPa is similar for the diamond ball milled and Ti-doped material. Diamond ball milling NaAlH4 with excess Al did not improve the rehydrogenation kinetics.
- Meisner,Tibbetts,Pinkerton,Olk,Balogh
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p. 254 - 263
(2008/10/08)
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- In-situ X-ray diffraction study of the decomposition of NaAlH4
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Phase transitions and crystal structure modifications were observed during the thermal-desorption decomposition of the alanate NaAlH4. This was accomplished through the use of in-situ X-ray powder diffraction. A sequence of θ-2θ scans were collected while heating the sample under a vacuum. The resulting diffraction patterns were assembled to provide a real-time representation of the decomposition reactions. It was found that upon heating, NaAlH4 initially experienced a lattice expansion principally in the c-axis direction. This was followed by continuous structural distortions observed as erratic variations in the Bragg intensities. Melting of NaAlH4 was observed at 180 °C followed by the rapid precipitation of a cubic Na3AlHx phase. This phase then underwent a slow transformation into a Na-rich cubic phase. The decomposition of NaAlH4 doped with a double catalyst (Ti+Zr) was also investigated. The uncatalyzed sample showed no decomposition when held under a vacuum at 150 °C for several hours. The catalyzed sample, on the other hand, began to decompose readily into the monoclinic (α)-Na3AlH6 phase when heated to 100 °C in vacuum. At 150 °C the (α)-Na3AlH6 phase decomposed in a second reaction according to α-Na3AlH6→3 NaH+Al+3/2 H2. The two decomposition reactions appear to be interdependent as the second transformation commenced only after the first reaction neared completion. The observed growth of a narrow Al (111) diffraction peak indicates the formation of aluminum crystallites (>100 nm) as a part of the decomposition reactions. This, and the fact that this solid state process is assisted through the interaction of a surface catalyst suggests long-range transport of a metal species. Some mechanisms are proposed to explain the catalytically enhanced kinetics of these materials.
- Gross, Karl J.,Guthrie, Steve,Takara, Satoshi,Thomas, George
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p. 270 - 281
(2008/10/08)
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- Sodium alanates for reversible hydrogen storage
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In this paper we show that sodium alanates may be used for reversible hydrogen storage, with the advantage of having high storage capacity combined with low cost. Both NaAlH4 and Na3AlH6 have been investigated for this application, and two complementary techniques have been used: improvement of the reaction kinetics by mechanical grinding, and chemical modification of the alloys. By these methods remarkable desorption/absorption kinetics are obtained. Sodium alanates so modified are capable of reversible hydrogen storage at the relatively low temperatures of around 80-140 °C, with a capacity of between 2.5 and 3.0 wt.%. The hydrides have an even higher reversible capacity of about 4.5-5 wt.% when operated at temperatures around 150-180 °C.
- Zaluska,Zaluski,Stroem-Olsen
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p. 125 - 134
(2008/10/08)
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- Homogeneous catalysis by evaporated solids
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About 10 years ago, sodium aluminate has been found as suitable catalyst for the pure dehydrogenation of methanol to anhydrous formaldehyde. At first, the reaction was supposed to follow a heterogeneous mechanism. Later on, experimental results with a special set-up were at variance. Recent investigations revealed the loss of catalytically active species from the solid aluminate into the vapour phase where the entire reaction is likely to take place. Furthermore, evaporated elemental sodium catalyses the dehydrogenation of methanol in a homogeneous vapour-phase reaction. With respect to additional investigations carried out lately, a conclusive reaction mechanism is proposed which explains both the reaction with sodium aluminate and evaporated elemental sodium as catalysts in a proper way.
- Ruf, Steffen,Emig, Gerhard
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p. 271 - 278
(2008/10/09)
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- Reactions of Sodium and Potassium Tetrahydroborates with Triethylenediamine in Diethyl Ether
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Reactions of sodium and potassium tetrahydroborates with triethylenediamine (TEDA) in diethyl ether is studied. Insoluble in diethyl ether, the tetrahydroborates pass into solution by forming an MBH4 · TEDA complex which, if heated, decomposes to aminoborane and a binary metal hydride. The compounds obtained were characterized by chemical analysis, X-ray diffraction, DTA, IR spectroscopy, and thin-layer chromatography.
- Mal'tseva,Golovanova
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p. 1221 - 1224
(2008/10/08)
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- Highly active alkali metal hydrides; their catalytic syntheses and properties
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Highly active alkali metal hydrides (M = Li, Na and K) can be prepared easily by the direct hydrogenation of alkali metals catalyzed by TiCl4 and naphthalene under mild conditions. The reaction scheme of this catalytic reaction is discussed. The hydrides obtained are in the form of a highly dispersed powder with an average primary particle size around 20 nm. The synthetic utility of these hydrides has been explored. Two of the examples are described.
- Zhang, Yiping,Liao, Shijian,Xu, Yun
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p. 211 - 222
(2008/10/08)
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- Nascent rotational quantum state distribution of NaH (NaD) from the reaction of Na*(42P) with H2, D2, and HD
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The nascent rotational quantum state distributions of NaH and NaD products resulting from the reactions of Na*(42Pj) with H2, D2, and HD have been determined using the laser pump-probe technique.We have observed a bimodal rotational distribution with a minor component peaking at low J and a major component peaking at high J.We have observed no evidence for a kinematic isotope effect on the product distribution.Our results are consisent with a model wherein the reaction occurs predominantly on the attractive 2B2 potential energy surface in near C2v geometry with the rotational distribution being determined late in the exit channel.
- Bililign, Solomon,Kleiber, P. D.
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p. 213 - 217
(2007/10/02)
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- Complex platinum hydride A2PtH4 with A=Na, K, Rb, or Cs
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Complex platinum hydrides have been synthesized by the reaction of binary alkali-metal hydrides with platinum sponge in a hydrogen atmosphere. The crystal structure was determined by X-ray investigations on powdered samples and neutron diffraction experiments on the deuterated compounds. The high temperature modifications crystallize in the K2PtCl6-type structure with hydrogen occupying two thirds of the chlorine positions. The tetragonal structures of the low temperature modifications are characterized by isolated, square planar [PtH4]2- groups.
- Bronger, W.,Auffermann, G.,Mueller, P.
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p. 243 - 252
(2008/10/08)
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