TY - JOUR
T1 - Thermodynamic modeling of the Na-Al-Ti-H system and Ti dissolution in sodium alanates
AU - Qiu, Caian
AU - Opalka, Susanne M.
AU - Løvvik, Ole M.
AU - Olson, Gregory B.
N1 - Funding Information:
This work was financially supported by the US Department of Energy under contract DE-FC04-02AL67610, managed by United Technology Research Center, East Hartford, Connecticut, USA.
PY - 2008/12
Y1 - 2008/12
N2 - Thermodynamic assessments were made to optimize thermodynamic models and parameter fits to selected experimental and first principles hypothetical predicted phase data within the Na-Al-Ti-H system. This enabled thermodynamic modeling of Ti solubility within the sodium alanates: NaAlH4 and Na3AlH6, and the relative stability of Ti-bearing phases. The modeling provides insights into the role of Ti originating from Ti-based activating agents commonly referred to as 'catalysts' in promoting reversibility of the Na-Al-H dehydrogenation and rehydrogenation reactions under moderate temperature and pressure conditions relevant to H storage applications. Preliminary assessments were made to evaluate H solubility in bcc-Ti and hcp-Ti, and stability of the hydride δ-TiH2. To model possible Ti dissolution in NaAlH4 and α-Na3AlH6, sub-lattice models were applied. A repulsive interaction is predicted by first principles calculations when Ti is dissolved in NaAlH4 or α-Na3AlH6, which becomes stronger with increasing temperature. Although Ti is virtually insoluble in NaAlH4 or α-Na3AlH6, a small addition of TiCl3 will induce a thermodynamic driving force for formation of TiH2 and/or TiAl3. The addition of pure Ti shows a weaker effect than TiCl3 and leads to formation of TiH2 only. Based on a combined interpretation of present thermodynamic modeling and prior experimental observations, the TiAl3 and TiH2 phases are ascribed to have a catalytic effect, not a thermodynamic destabilization effect, on the reversibility of the dehydrogenation/rehydrogenation reactions in the Na-Al-H system.
AB - Thermodynamic assessments were made to optimize thermodynamic models and parameter fits to selected experimental and first principles hypothetical predicted phase data within the Na-Al-Ti-H system. This enabled thermodynamic modeling of Ti solubility within the sodium alanates: NaAlH4 and Na3AlH6, and the relative stability of Ti-bearing phases. The modeling provides insights into the role of Ti originating from Ti-based activating agents commonly referred to as 'catalysts' in promoting reversibility of the Na-Al-H dehydrogenation and rehydrogenation reactions under moderate temperature and pressure conditions relevant to H storage applications. Preliminary assessments were made to evaluate H solubility in bcc-Ti and hcp-Ti, and stability of the hydride δ-TiH2. To model possible Ti dissolution in NaAlH4 and α-Na3AlH6, sub-lattice models were applied. A repulsive interaction is predicted by first principles calculations when Ti is dissolved in NaAlH4 or α-Na3AlH6, which becomes stronger with increasing temperature. Although Ti is virtually insoluble in NaAlH4 or α-Na3AlH6, a small addition of TiCl3 will induce a thermodynamic driving force for formation of TiH2 and/or TiAl3. The addition of pure Ti shows a weaker effect than TiCl3 and leads to formation of TiH2 only. Based on a combined interpretation of present thermodynamic modeling and prior experimental observations, the TiAl3 and TiH2 phases are ascribed to have a catalytic effect, not a thermodynamic destabilization effect, on the reversibility of the dehydrogenation/rehydrogenation reactions in the Na-Al-H system.
KW - Direct method lattice dynamics
KW - Phase diagrams
KW - Sodium alanates
KW - Thermodynamic modeling
KW - Titanium dissolution
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U2 - 10.1016/j.calphad.2008.08.005
DO - 10.1016/j.calphad.2008.08.005
M3 - Article
AN - SCOPUS:55549123494
SN - 0364-5916
VL - 32
SP - 624
EP - 636
JO - Calphad: Computer Coupling of Phase Diagrams and Thermochemistry
JF - Calphad: Computer Coupling of Phase Diagrams and Thermochemistry
IS - 4
ER -