Characterization of the Local Titanium Environment in Doped Sodium Aluminum Hydride using X-ray Absorption Spectroscopy
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Characterization of the Local Titanium Environment in Doped Sodium Aluminum Hydride using X-ray Absorption Spectroscopy J. Graetz, A.Yu. Ignatov*, T.A. Tyson*, J.J. Reilly, J. Johnson Department of Energy Sciences and Technology, Brookhaven National Laboratory, Upton, New York 11973 *Department of Physics, New Jersey Institute of Technology, Newark, New Jersey 07102 ABSTRACT Ti K-edge x-ray absorption spectroscopy was used to explore the local titanium environment and valence in 2-4 mol% Ti-doped sodium alanate. An estimate of the oxidation state of the dopant, based upon known standards, revealed a zero-valent titanium atom. An analysis of the near-edge and extended fine structures indicates that the Ti does not enter substitutional or interstitial sites in the NaAlH4 lattice. Rather, the Ti is located on/near the surface and is coordinated by 10.2±1 aluminum atoms with an interatomic distance of 2.82±0.01 Å, similar to that of TiAl3. The Fourier transformed EXAFS spectra reveal a lack of long-range order around the Ti dopant indicating that the Ti forms nano-clusters of TiAl3. The similarity of the spectra in the hydrided and dehydrided samples suggests that the local Ti environment is nearly invariant during hydrogen cycling. INTRODUCTION The demonstration of reversible hydrogen cycling in Ti-doped sodium aluminum hydride [1] has generated considerable interest in the sodium alanates. Since this discovery there has been a number of studies focused on improving the catalytic effects and understanding the role of the dopant in the sodium alanates. Despite this tremendous effort, the mechanism by which NaAlH4 is activated in the presence of a small amount of a transition metal is still not well understood. In part, this is because the location and valence of the activating species is unknown. The reversible decomposition of sodium aluminum hydride occurs through the following twostep reaction: NaAlH4 ↔ 1/3Na3AlH6 + 2/3Al+H2 ↔ NaH + Al + 3/2H2,
(1)
yielding a theoretical hydrogen capacity of 5.6 wt%. There are a number of possible mechanisms by which a metal dopant (e.g. Ti) might enhance the dehydriding kinetics of reaction 1, such as assisting the conversion of atomic hydrogen into molecular hydrogen at the surface. However, this situation is unlikely due to the strong thermodynamic driving force on the Ti to form a hydride in the presence of the desorbed hydrogen. Another proposed mechanism involves the substitution of the dopant into the lattice. X-ray diffraction studies of Ti and Zr-doped NaAlH4 have revealed lattice distortions associated with doping NaAlH4, suggesting a bulk substitution of Ti for Na [2, 3]. The enhanced kinetics is attributed to vacancy formation induced by the insertion of Ti3+ or Ti4+ into the lattice. Another possibility is that the hydrogen diffuses to the surface in an Al-H complex where it is dissociated by the catalyzing agent and releases H2. This process leads to a significant coalescence of Al on the particle surface in the dehydrided state. Energy dispersive spectroscopy
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