Kinetic Study and Determination of the Enthalpies of Activation of the Dehydrogenation of Titanium- and Zirconium-Doped NaAlH₄ and Na₃AlH₆

Abstract

The rates of the dehydrogenation of the sodium alanates NaAlH4 and Na3AlH6 doped with 2 mol % Ti or Zr have been measured over the temperature range 363−423 K. NaAlH4 and Na3AlH6 undergo dehydrogenation at equal rates upon direct doping with titanium. However, Na3AlH6 arising from the dehydrogenation of Ti-doped NaAlH4 undergoes dehydrogenation at much slower rates. Rate constants were determined from the slopes of the dehydrogenation profiles. On the basis of Eyring theory, the enthalpies of activation, ΔH ⧧, for the dehydrogenation reactions were determined to be ∼100 kJ·mol-1 for both Ti-doped NaAlH4 and Na3AlH6 and ∼135 kJ·mol-1 for both Zr-doped NaAlH4 and Na3AlH6. These results suggest that the dehydrogenation reaction pathways are highly sensitive to the nature and distribution of the dopant but not to differences in the Al−H bonding interactions in [AlH4]- and [AlH6]3- complex anions. Furthermore, we conclude that the kinetics are probably influenced by processes such as nucleation and growth and/or range atomic transport phenomenon.

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

“…Note that such an effect is less (or not) apparent in uncycled (doped) samples. This supports the idea that titanium diffusion and some 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 [10,12,13,38]. However, before reaching a final conclusion and a more quantitative assessment of the various effects more systematic studies of doping and cycling are needed.…”

“…One school of thought has held that the remarkable enhancement of the hydrogen cycling kinetics in Ti doped NaAlH 4 is due to surface-localized catalytic species consisting of elemental titanium or a Ti-Al alloy [1,4,6,10]. Alternatively, it has been hypothesized that doping involves the substitution of titanium into the bulk of the hydride [11][12][13]. The present work represents an attempt to measure possible structural and microstructural changes in NaAlH 4 induced by milling, doping and cycling.…”

Structure and hydrogen dynamics of pure and Ti-doped sodium alanate

“…It was recently reported that a few percent of Ti doping in NaAlH 4 renders accelerated and reversible hydrogen release under moderate conditions [1]. In spite of the extensive investigations of Ti-doped NaAlH 4 that have resulted, little is known about the mechanism by which Ti enhances the cycling kinetics of hydrogen [2,3]. In fact, even the location of the Ti atoms remains unclear.…”

“…This clearly indicates that the presence of Ti could facilitate breaking of the Al-H bond. is the cohesive energy obtained by allowing the atoms to relax but imposing the NaAlH4 relaxed supercell; E atom coh (SP) is the same but obtained from spin-polarized calculations; E full coh is the result obtained when allowing both atoms and cell to relax, and V the resulting volume of the 96-atom supercell inÅ 3 . In the Ti→Al case (Fig.…”

Li–Na ternary amidoborane for hydrogen storage: experimental and first-principles study

“…The search for efficient, safe and economically viable hydrogen storage materials represents great challenges to the scientific society. In the past few years, tremendous efforts have been devoted to the investigation of light-weighted chemical and complex hydrides, such as amide-hydrides, [1][2][3] borohydrides, [4][5][6] alanates [6][7][8][9][10] and ammonia borane. [11][12][13] Ammonia borane (NH 3 BH 3 ) possesses a high gravimetric hydrogen capacity (19.6 wt% H 2 ); however, its stepwise kinetic barrier borne dehydrogenation together with the toxic by-product borazine impedes its practical application in proton exchange membrane (PEM) fuel cells.…”