Decomposition Behavior of Ti-doped NaAlH 4 Studied using X-ray Absorption Spectroscopy at the Titanium K-edge
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Decomposition Behavior of Ti-doped NaAlH4 Studied using X-ray Absorption Spectroscopy at the Titanium K-edge E. Bruster1, T. A. Dobbins1, R. Tittsworth2, D. Anton3 1
Lousiana Tech University, Institute for Micromanufacturing, P.O. Box 10137 Ruston, LA 71272, U.S.A. 2
Louisiana State University Center for Advanced Microstructures and Devices, 6980 Jefferson Hwy., Baton Rouge, LA 70806, U.S.A. 3
United Technologies Research Center 411 Silver Ln. E. Hartford, CT 06108, U.S.A. ABSTRACT The local bonding environment of Ti3+-dopant atoms in NaAlH4 after decomposition to release H2 has been studied using x-ray absorption fine structure (XAFS). The titanium K-edge spectra from doped hydride samples and the standard materials TiCl3 and TiO2 were collected in ambient atmosphere at the synchrotron source at the Center for Advanced Microstructures and Devices (CAMD). Titanium valence states present in the spectra collected from Ti-doped NaAlH4 after decomposition in air are Ti3+ and Ti4+. The Ti3+ is attributed to unreacted TiCl3. The Ti4+ present in the sample is attributed to TiO2 occurring after air oxidation. Coupled with studies of the kinetics of hydrogen desorption reactions, examination of dopant ion valence states after entry into the lattice may lead to better understanding of the interrelationship between lattice doping and desorption kinetics. INTRODUCTION Complex metal hydrides are a class of materials which crystallize into compounds having H ions incorporated into interstitial lattice sites. These materials undergo thermal decomposition to release H2 gas at elevated temperatures. After much research on the reactivity of complex metal hydrides —it is presently understood that metallic dopant atom additions play a critical role in catalyzing their reversible hydrogen sorption1. In order for complex metal hydrides to meet DoE stated targets for hydrogen storage materials—a precise understanding of the role these dopant catalysts must be understood. A recent study has shown, systematically, those transition and rare earth metal cations that offer enhanced desorption rates in NaAlH4. The results of desorption kinetics as a function of ionic radii are summarized in Figure 1.2 Studies have characterized dopant effects in Ti-doped NaAlH4—resulting in several proposed mechanisms.3,4,5,6,7 Some of these studies report Na+ substitution by titanium dopants.3,6,7 Iniguez et al. have reported that Ti entry into the lattice via Na sites. Titanium being a strong hydrogen attractor, weakening Al-H bonds is reported to be responsible for enhanced hydrogen desorption kinetics.4 Graetz et al. used x-ray absorption spectroscopy and report the formation of amorphous TiAl3—thus implying Al-H bond weakening, as well.5 -
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D.L. Anton, "Hydrogen Desorption Kinetics in Transition Metal Modified NaAlH4", Journal of Alloys and Compounds V356-357 (2003) 400-404.
8.00 Desorption Rate (5 Min) - wt% per h 7.00 *
6.00 5.00 4.00 3.00 2.00 1.00
Zr4+
Zn2+
V4+
Yb3+
V3+
V2+
Ti3+
Ti4+
Ti3+
Ti2+
Sr2+
Ru3+
Pt4+
Rh3+
Pd
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