New Possibilities for Understanding Complex Metal Hydrides via Synchrotron X-ray Studies: Making Synchrotron X-ray Characterization relevant to the Hydrogen Economy

Tabbetha A. Dobbins (Louisiana Tech University)

This research seeks to understand the role of catalysts in hydride structures. X-ray absorption spectroscopy is used to examine the chemical interaction of catalysts (TiCl3) with the host hydride powder (NaAlH4). Results show that TiCl3 transforms first to metallic Ti.then reacts with Al3+ in NaAlH4 to form TiAlx complexes. It remains unclear, however, whether the formation of those TiAlx complexes is directly related to catalysis of the H2 desorption reaction in the hydride. Ultrasmall angle x-ray scattering (USAXS) is used to study trends in powder surface area with various treatments. Powder surface area is an indicator of hydride particle damage accumulation (i.e. crack formation in and fracture of the particle) during high energy ball milling to add catalysts and during hydrogen desorption. Sodium alanate (NaAlH4) powders are routinely high energy milled with the transition metal salt catalysts, TiCl2, TiCl3, ZrCl4, and VCl3. Our research studies the variation in powder surface area with milling time and with desorption treatment. Results show that NaAlH4 has an inherently high surface area (characterized by the surface fractal morphology) and that after catalyst introduction by high energy ball milling, higher surface areas are retained for catalysts which show the best desorption kinetics and lowest surface areas are yielded for poorly performing catalysts. After desorption, the surface area was lower, relative to the undesorbed powders, in all cases. Variation in powder surface area is attributed to long-range diffusion in the hydride powders. Transition metal salts may play a role in enhancing or inhibiting diffusion in the host material. Combined, these synchrotron X-ray studies (i.e. absorption spectroscopy and scattering) attempt to differentiate between chemical contributions of the transition metal salt (i.e. acting as catalyst for chemical reactions) and transport kinetic contributions (i.e. acting to promote higher diffusivity).

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