Thermodynamic modeling of the Na-Al-Ti-H system and Ti dissolution in sodium alanates

Caian Qiu*, Susanne M. Opalka, Ole M. Løvvik, Gregory B. Olson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)624-636
Number of pages13
JournalCalphad: Computer Coupling of Phase Diagrams and Thermochemistry
Volume32
Issue number4
DOIs
StatePublished - Dec 1 2008

Keywords

  • Direct method lattice dynamics
  • Phase diagrams
  • Sodium alanates
  • Thermodynamic modeling
  • Titanium dissolution

ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Computer Science Applications

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