Active Ti Species in TiCl3-Doped NaAlH4. Mechanism for Catalyst Deactivation

Abstract

The nature of the active Ti species in TiCl3-doped NaAlH4, a promising hydrogen storage material, was studied as a function of the desorption temperature with Ti K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, Ti K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and X-ray diffraction (XRD). In the freshly prepared sample, Ti was amorphous and surrounded by 4.8 Al atoms divided between two shells at 2.71 and 2.89 Å. In the next shell, 1.9 Ti atoms were detected at 3.52 Å. It was concluded that 30% of Ti was incorporated into the surface of Al crystallites and 70% of Ti occupied interstitials in the NaAlH4 lattice, possibly forming trimeric, triangular Ti entities. After hydrogen desorption at 125 C, NaAlH4 decomposed and the Ti-Al coordination number increased from 4.8 to 8.5. We propose that all Ti is incorporated into the surface layer of the formed Al. After the material was heated to 225 C, the local structure of Ti, as inferred from EXAFS and XANES spectroscopy, was identical to the local structure of a TiAl3 alloy. However, the formed alloy was amorphous and was only detected in XRD by an increase of the background intensity around the Al diffraction. These so-called "TiAl3 clusters" agglomerated in the heat treatment to 475 C, forming crystalline TiAl3. Earlier work has shown that increasing the desorption temperature of NaAlH4 lowers the absorption rate and capacity of hydrogen in the next step. Thus, by comparing our results with absorption properties published in the literature on similar samples, we could rank the activity of the Ti for hydrogen absorption as Ti in the Al surface > TiAl3 cluster > crystalline TiAl3, therewith indicating that Ti incorporated into the surface of Al is the most active for the absorption of hydrogen.