Understanding the Role of Ti in Reversible Hydrogen Storage as Sodium Alanate:  A Combined Experimental and Density Functional Theoretical Approach

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Abstract

We report the results of an experimental and theoretical study of hydrogen storage in sodium alanate (NaAlH₄). Reversible hydrogen storage in this material is dependent on the presence of 2−4% Ti dopant. Our combined study shows that the role of Ti may be linked entirely to Ti-containing active catalytic sites in the metallic Al phase present in the dehydrogenated NaAlH₄. The EXAFS data presented here show that dehydrogenated samples contain a highly disordered distribution of Ti−Al distances with no long-range order beyond the second coordination sphere. We have used density functional theory techniques to calculate the chemical potential of possible Ti arrangements on an Al(001) surface for Ti coverages ranging from 0.125 to 0.5 monolayer (ML) and have identified those that can chemisorb molecular hydrogen via spontaneous or only moderately activated pathways. The chemisorption process exhibits a characteristic nodal symmetry property for the low-barrier sites:  the incipient doped surface-H₂ adduct's highest occupied molecular orbital (HOMO) incorporates the σ* antibonding molecular orbital of hydrogen, allowing the transfer of charge density from the surface to dissociate the molecular hydrogen. This work also proposes a plausible mechanism for the transport of an aluminum hydride species back into the NaH lattice that is supported by Car−Parrinello molecular dynamics (CPMD) simulations of the stability and mobility of aluminum clusters (alanes) on Al(001). As an experimental validation of the proposed role of titanium and the subsequent diffusion of alanes, we demonstrate experimentally that AlH₃ reacts with NaH to form NaAlH₄ without any requirement of a catalyst or hydrogen overpressure.

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This article has been cited by 26 ACS Journal articles (5 most recent appear below).

• 

  New Aspects on the Decomposition of Sodium Alanate Revealed by Small-Angle X-ray Scattering

  Sabrina Sartori, Kenneth D. Knudsen, and Bjørn C. Hauback
  The Journal of Physical Chemistry C2012 Article ASAP

  • New Aspects on the Decomposition of Sodium Alanate Revealed by Small-Angle X-ray Scattering

    Sabrina Sartori, Kenneth D. Knudsen, and Bjørn C. Hauback
    The Journal of Physical Chemistry C2012 Article ASAP

    Transition metals added to sodium aluminum hydride by high-energy ball milling have been shown to significantly enhance its absorption and desorption properties. In the present study, we have used small-angle X-ray scattering to elucidate how TiCl3 ...

    Abstract | Full Text HTML | Hi-Res PDF | PDF w/ Links
• 

  Proposed Mechanisms for the Catalytic Activity of Ti in NaAlH₄

  Terry J. Frankcombe
  Chemical Reviews2011 Article ASAP

  • Proposed Mechanisms for the Catalytic Activity of Ti in NaAlH₄

    Terry J. Frankcombe
    Chemical Reviews2011 Article ASAP
    Abstract | Full Text HTML | Hi-Res PDF | PDF w/ Links
• 

  Effect of Titanium doping of Al(111) surfaces on alane formation, mobility and desorption.

  Irinder S Chopra , Santanu Chaudhuri , Jean-Francois Veyan , Jason Graetz , and Yves J. Chabal
  The Journal of Physical Chemistry C 0 (ja),

  • Effect of Titanium doping of Al(111) surfaces on alane formation, mobility and desorption.

    Irinder S Chopra , Santanu Chaudhuri , Jean-Francois Veyan , Jason Graetz , and Yves J. Chabal
    The Journal of Physical Chemistry C 0 (ja),

    Alanes are critical intermediates in hydrogen storage reactions for mass transport during the formation of complex metal hydrides. Titanium has been shown to promote hydrogen desorption and hydrogenation, but its role as a catalyst is not clear. Combining ...

    Abstract | Hi-Res PDF | PDF w/ Links
• 

  ²⁷Al, ²³Na, and ⁴⁵Sc Solid-State NMR Studies of ScCl₃-Doped NaAlH₄

  Margriet H.W. Verkuijlen, P. Jan M. van Bentum, Oleg Zabara, Maximilian Fichtner, and Arno P. M. Kentgens
  The Journal of Physical Chemistry C2011 Article ASAP

  • ²⁷Al, ²³Na, and ⁴⁵Sc Solid-State NMR Studies of ScCl₃-Doped NaAlH₄

    Margriet H.W. Verkuijlen, P. Jan M. van Bentum, Oleg Zabara, Maximilian Fichtner, and Arno P. M. Kentgens
    The Journal of Physical Chemistry C2011 Article ASAP

    The exact role of catalysts, such as titanium, scandium, or cerium, in improving the hydrogen kinetics in NaAlH4 is not completely understood. Here, we studied the local structure of scandium and the formation of different species in ScCl3-doped NaAlH4 by ...

    Abstract | Full Text HTML | Hi-Res PDF | PDF w/ Links
• 

  Regeneration of Aluminum Hydride Using Trimethylamine

  David Lacina, James Reilly, Yusuf Celebi, James Wegrzyn, John Johnson, and Jason Graetz
  The Journal of Physical Chemistry C2011 115 (9), 3789-3793

  • Regeneration of Aluminum Hydride Using Trimethylamine

    David Lacina, James Reilly, Yusuf Celebi, James Wegrzyn, John Johnson, and Jason Graetz
    The Journal of Physical Chemistry C2011 115 (9), 3789-3793

    Aluminum hydride is an attractive reducing agent and energy storage compound possessing a low decomposition temperature and a high gravimetric and volumetric hydrogen density. However, it is thermodynamically unstable at room temperature and requires ...

    Abstract | Full Text HTML | Hi-Res PDF | PDF w/ Links

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