Synchrotron X-ray and neutron diffraction studies of NaAlH4 containing Ti additives

Brinks, H.W.; Jensen, Craig M.; Srinivasan, Sesha S.; Hauback, Bjørn C.; Blanchard, Didier; Murphy, Keeley

doi:10.1016/j.jallcom.2003.12.024

Cited by 155 publications

(184 citation statements)

References 21 publications

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“…The cell parameters at 10 K and room temperature are consistent with those reported in previous work [11,14,[24][25][26][27][28]. However, they show small but significant differences among the samples (up to 30 e.s.d.…”

Section: Structuresupporting

confidence: 91%

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

Gomes

Renaudin

Hagemann

et al. 2005

Journal of Alloys and Compounds

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The effects of milling and doping NaAlH 4 with TiCl 3 , TiF 3 and Ti(OBu n ) 4 , and of cycling doped NaAlH 4 have been investigated by infrared (IR) and Raman spectroscopy and X-ray powder diffraction. Milling and doping produce similar effects. Both decrease the crystal domain size (∼900Å for milled and ∼700Å for doped, as compared to ∼1600Å for unmilled and undoped NaAlH 4 ) and increase anisotropic strain (by a factor >2.5, mainly along c). They also influence structure parameters such as the axial ratio c/a, cell volume and atomic displacement amplitudes. They show IR line shifts by ∼15 cm −1 to higher frequencies for the Al-H asymmetric stretching mode ν 3 , and by ∼20 cm −1 to lower frequencies for one part of the H-Al-H asymmetric bending mode ν 4 , thus suggesting structural changes in the local environment of the [AlH 4 ] − units. The broad ν 3 bands become sharpened which suggests a more homogeneous local environment of the [AlH 4 ] − units, and there appears a new vibration at 710 cm −1 . The Raman data show no such effects. Cycling leads to an increase in domain size (1200-1600Å), IR line shifts similar to doped samples (except for TiF 3 : downward shift by ∼10 cm −1 ) and a general broadening of the ν 3 mode that depend on the nature of the dopants. These observations support the idea that some Ti diffusion and substitution into the alanate lattice does occur, in particular during cycling, and that this provides the mechanism through which Ti-doping enhances kinetics during re-crystallisation.

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Section: Structuresupporting

confidence: 91%

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

Gomes

Renaudin

Hagemann

et al. 2005

Journal of Alloys and Compounds

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“…This accords with previous reports that the NaAlH 4 obtained from the hydrogenation of stoichiometric amounts NaH and Al* contains significant amounts of residual Al and Na 3 AlH 6 owing to the formation of an Al 0.85 Ti 0.15 solid solution. 15,16 Accurate quantification of the Al species present in the reaction mixtures is not feasible as Al in the probe contributes (albeit minimally) to the intensity of the Al metal resonance. Moreover, the resonance for alumina contained in the tube (3) overlaps with the alanate peaks.…”

Section: Q4mentioning

confidence: 99%

In situ high pressure NMR study of the direct synthesis of NaAlH4

Humphries

Birkmire

Hauback

et al. 2013

Phys. Chem. Chem. Phys.

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The direct synthesis of NaAlH 4 has been studied, for the first time, by in situ 27 Al and 23 Na wide-line NMR spectroscopy using high pressure NMR apparatus. Na 3 AlH 6 formation is observed within two minutes of hydrogenation addition, while NaAlH 4 is detected after a total of four minutes. This indicates the formation of the hexahydride does not proceed to completion before the formation of the tetrahydride ensues.The practical utilization of hydrogen as an energy carrier awaits the development of high-capacity, hydrogen storage materials that can be recharged under moderate conditions. A viable on-board hydrogen carrier must have high gravimetric and volumetric hydrogen capacities; thermodynamic properties that fall within rather stringent limits; and dehydrogenation and rehydrogenation kinetics that allow hydrogen cycling at moderate temperatures and pressures. 1,2 One of the most important breakthroughs in the development of hydrogen storage materials in the past 20 years was provided by Q4Bogdanović and Schwickardi, whose pioneering studies demonstrated that addition of selected titanium compounds to NaAlH 4 results in enhanced kinetics and reversibility under moderate conditions in the solid state. 3 These studies were prompted by earlier reports from Wiberg et al., who observed that titanium compounds catalyze the dehydrogenation of complex aluminum hydrides in solution. 4 The enigmatic extension of this catalytic effect to the solid state has been the inspiration for over 260 publications on Ti-enhanced NaAlH 4 alone, and it is safe to estimate that it prompted an equal number of studies of the effect of Ti additives on the dehydrogenation kinetics of other complex hydrides. 5

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“…Some authors claim that Ti alloys with Al as an amorphous species upon doping TiCl 3 with NaAlH 4 , 9,[17][18][19] while others observed the formation of metallic Ti clusters. 20 Jensen et al claim on the basis of an IR spectroscopy study that Ti is located in the vicinity of the AlH 4 -unit and influences the Al-H asymmetric stretching frequency.…”

Section: Introductionmentioning

confidence: 99%

Active Ti Species in TiCl₃-Doped NaAlH₄. Mechanism for Catalyst Deactivation

et al. 2007

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The nature of the active Ti species in TiCl 3 -doped NaAlH 4 , a promising hydrogen storage material, was studied as a function of the desorption temperature with Ti K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, Ti K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and X-ray diffraction (XRD). In the freshly prepared sample, Ti was amorphous and surrounded by 4.8 Al atoms divided between two shells at 2.71 and 2.89 Å. In the next shell, 1.9 Ti atoms were detected at 3.52 Å. It was concluded that 30% of Ti was incorporated into the surface of Al crystallites and 70% of Ti occupied interstitials in the NaAlH 4 lattice, possibly forming trimeric, triangular Ti entities. After hydrogen desorption at 125°C, NaAlH 4 decomposed and the Ti-Al coordination number increased from 4.8 to 8.5. We propose that all Ti is incorporated into the surface layer of the formed Al. After the material was heated to 225°C, the local structure of Ti, as inferred from EXAFS and XANES spectroscopy, was identical to the local structure of a TiAl 3 alloy. However, the formed alloy was amorphous and was only detected in XRD by an increase of the background intensity around the Al diffraction. These so-called "TiAl 3 clusters" agglomerated in the heat treatment to 475°C, forming crystalline TiAl 3 . Earlier work has shown that increasing the desorption temperature of NaAlH 4 lowers the absorption rate and capacity of hydrogen in the next step. Thus, by comparing our results with absorption properties published in the literature on similar samples, we could rank the activity of the Ti for hydrogen absorption as Ti in the Al surface > TiAl 3 cluster > crystalline TiAl 3 , therewith indicating that Ti incorporated into the surface of Al is the most active for the absorption of hydrogen.

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Synchrotron X-ray and neutron diffraction studies of NaAlH4 containing Ti additives

Cited by 155 publications

References 21 publications

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

In situ high pressure NMR study of the direct synthesis of NaAlH4

Active Ti Species in TiCl₃-Doped NaAlH₄. Mechanism for Catalyst Deactivation

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Synchrotron X-ray and neutron diffraction studies of NaAlH4 containing Ti additives

Cited by 155 publications

References 21 publications

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

In situ high pressure NMR study of the direct synthesis of NaAlH4

Active Ti Species in TiCl3-Doped NaAlH4. Mechanism for Catalyst Deactivation

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Active Ti Species in TiCl₃-Doped NaAlH₄. Mechanism for Catalyst Deactivation