Evolution of the Local Structure around Ti Atoms in NaAlH₄ Doped with TiCl₃ or Ti₁₃·6THF by Ball Milling Using X-ray Absorption and X-ray Photoelectron Spectroscopy

Abstract

X-ray absorption and X-ray photoelectron spectroscopy are used to investigate NaAlH4 doped with 5 mol % of Ti on the basis of either TiCl3 or Ti13.6THF by ball milling. X-ray photoelectron spectroscopy (XPS) analysis of TiCl3 or Ti colloid doped samples indicates that Ti species do not remain on the sample surface but are driven into the material with increasing milling time. The surface concentration of Ti continues to decrease during subsequent cycles under hydrogen. After several cycles, it reaches a constant value of 0.5 at. % independently of the nature of the precursor. Moreover, metallic aluminum is already present at the surface after 2 min of ball milling in the case of TiCl3 doped Na-alanate, whereas it is totally absent in the case of Ti colloid doped samples at any milling time. Upon cycling, the atomic concentration of metallic Al at the surface evolves with the reaction under hydrogen, in contrast to the Ti concentration. Analysis of the binding energies of samples doped with TiCl3 or Ti colloid, after eight desorption/absorption cycles, reveals that the Na, O, and Ti environment remains the same, while the Al environment undergoes changes. According to the extended X-ray absorption fine structure (EXAFS) analysis of TiCl3 doped Na-alanate, the local structure around Ti during the first cycle is close to that of metallic Ti but in a more distorted state. In the case of the Ti colloid doped sample, a stripping of the oxygen shell occurs. After eight cycles, a similar intermetallic phase between Ti and Al is present in the hydrogenated state of TiCl3 or Ti colloid doped samples. The local structure around Ti atoms after eight cycles consists of Al and Ti backscatterers with a Ti-Al distance of 2.79 angstroms and a Ti-Ti distance of 3.88 angstroms. This local structure is not exactly the TiAl3 phase because it differs significantly from the alloy phase in its fine structure and lacks long-range order. Volumetric measurements performed on these samples indicate that the formation of this local structure is responsible for the reduction of the reversible hydrogen capacity with the increasing number of cycles. Moreover, the formation of the alloy-like phase is correlated with a decrease of the desorption/absorption reaction rate.

Complex and liquid hydrides for energy storage

“…The role of the Ti as a catalyst is to increase the reaction rate for the hydrogen desorption and absorption reaction, i.e. to allow the reaction to equilibrate in the time frame of the experiment [178]. The hydrogen desorption reaction rate increases linearly as a function of the TiCl 3 that is added, up to 5 mol%; for larger quantities, the reaction rate does not increase anymore [176].…”

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

“…Some authors claim that Ti alloys with Al as an amorphous species upon doping TiCl 3 with NaAlH 4 , 9,[17][18][19] while others observed the formation of metallic Ti clusters. 20 Jensen et al claim on the basis of an IR spectroscopy study that Ti is located in the vicinity of the AlH 4 -unit and influences the Al-H asymmetric stretching frequency. 21 …”

Ultrasmall-angle X-ray scattering (USAXS) studies of morphological trends in high energy milled NaAlH4 powders

“…Since the discovery of catalytic effects in transition metal doped sodium alanate (NaAlH 4 ) powders by Bogdanovic and Schwickardi [1], dopant effects in these powders has been studied in an effort to understand and optimize doping strategies to control kinetic barriers to hydrogen release from alanate powders [2][3][4][5][6][7][8][9]. X-ray absorption spectroscopy has been used to characterize the local environment of dopant cations in efforts to elucidate dopant effects [2][3][4][5][6][7][8][9].…”