X-ray diffraction studies of titanium and zirconium doped NaAlH4: elucidation of doping induced structural changes and their relationship to enhanced hydrogen storage properties

doi:10.1016/S0925-8388(01)01955-7

Journal of Alloys and Compounds

Volume 337, Issues 1–2, 2 May 2002, Pages L8-L11

https://doi.org/10.1016/S0925-8388(01)01955-7 Get rights and content

Abstract

X-ray diffraction patterns of NaAlH₄ doped with up to 10 mol.% of either titanium or zirconium do not contain Bragg peaks for the bulk metals or their aluminum alloys. Instead the hydride lattice parameters a and c undergo significant contraction upon 2 mol.% doping and then expand as the doping level increases from 2 to 5 mol.%. These results are explained by a model that entails substitution of sodium cations by variable valence transition metal cations and the creation of Na⁺ vacancies in the bulk hydride lattice.

Introduction

Recently, Bogdanovic discovered that the elimination of hydrogen from solid NaAlH₄ is markedly accelerated and rendered reversible under moderate conditions upon mixing the hydride with a few mole percent of selected transition metal complexes [1]. This was a revolutionary finding in the area of metal hydrides as hydrogen cycling at moderate temperatures was unprecedented for saline hydrides. The hydrogen cycling performance of doped NaAlH₄ has since been the subject of intensive investigation [2], [3], [4], [5], [6], [7], [8], [9]. These studies have led to considerable practical improvements and it now appears that the doped hydride could possibly be developed as a viable means for the onboard storage of hydrogen [2], [3], [5], [6], [7], [8]. However, studies by Mössbauer spectroscopy [5], scanning electron microscopy [9], and in-situ X-ray diffraction [4] have not revealed a fundamental understanding of why the hydrogen cycling kinetics are enhanced upon doping the hydride. In order to gain insight into this phenomenon, we have conducted X-ray diffraction studies on samples NaAlH₄ containing variable levels of titanium and zirconium doping. Contrary to prior speculation that the dopants are segregated, surface localized species [1], [4], [5], [9], these studies indicate substitution of the dopants into the bulk hydride lattice.

Section snippets

Materials

Titanium n-butoxide, Ti(OBuⁿ)₄, and zirconium n-propoxide, Zr(OPrⁿ)₄, and sodium aluminum hydride, NaAlH₄, were purchased from Aldrich Chemical. The alkoxide complexes were used as received.

Purification of NaAlH₄

The purification procedure was carried out in a glove box under nitrogen. The aluminum present in the commercially obtained NaAlH₄ was removed by filtration following dissolution of the hydride in a minimal amount of tetrahydrofuran (THF). Analytically pure NaAlH₄ was precipitated upon the addition of

Results and discussion

Two series of samples of doped NaAlH₄ were prepared for X-ray diffraction (XRD) studies. The first was doped with titanium at the 2, 5, and 10 mol.% levels, while the second was doped with zirconium at the same mol.% levels. The XRD patterns of the doped materials and undoped NaAlH₄ were obtained upon Rietveld refinement of diffraction data that was collected at 25 °C. The lattice parameters were then calculated from the XRD patterns. The results of these studies are summarized in Table 1.

Conclusions

XRD studies of NaAlH₄ which was doped with 0–10 mol.% titanium and zirconium revealed that significant changes occur in the lattice parameters a and c upon doping. This clearly indicates that, contrary to earlier speculation, the dopants are substituted into the lattice of the bulk hydride. Thus the remarkable enhancement of dehydriding kinetics that arises upon doping NaAlH₄ is apparently associated with dopant induced lattice distortions rather than a catalytic effect. We have found an

Acknowledgments

This study was administered through the New Energy and Industrial Technology Development Organization of Japan (NEDO) as a part of the International Clean Energy Network using Hydrogen Conversion (WE-NET) program with funding from the Ministry of Economy, Trade and Industry of Japan (METI).

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LetterX-ray diffraction studies of titanium and zirconium doped NaAlH4: elucidation of doping induced structural changes and their relationship to enhanced hydrogen storage properties

Abstract

Introduction

Section snippets

Materials

Purification of NaAlH4

Results and discussion

Conclusions

Acknowledgments

J. Alloys Comp.

Int. J. Hydrogen Energy

J. Alloys Comp.

J. Alloys Comp.

J. Alloys Comp.

J. Alloys Comp.

Letter
X-ray diffraction studies of titanium and zirconium doped NaAlH₄: elucidation of doping induced structural changes and their relationship to enhanced hydrogen storage properties

Purification of NaAlH₄