Functionality of the nanoscopic crystalline Al/amorphous Al 50Ti 50 surface embedded composite observed in the NaAlH 4 + xTiCl 3 system after milling

Abstract

The NaAlH 4 + xTiCl 3 ( x < 0.1) system has been studied by a combination of X-ray synchrotron and neutron diffraction, and isotopic H 2/D 2 scrambling after the completion of the milling process, and the first thermal release of hydrogen (H). An in situ X-ray synchrotron diffraction study of the isochronal release of hydrogen from planetary milled (PM) NaAlH 4 + 0.1TiCl 3 shows that crystalline (c-) Al 1− x Ti x phases do not form until almost all H is released from the sample, demonstrating that the surface embedded nanoscopic crystalline Al/amorphous (a-) Al 50Ti 50 composite facilitates the release of H during the very first thermal desorption. Planetary milled (PM) NaAlH 4 + xTiCl 3 is observed to disproportionate at room temperature, with no NaAlH 4 remaining after ca. 200 days. A complete lack of ambient hydrogen release from PM NaAlH 4 + 0.1Al (80 nm) measured over 200 days suggests that the nanoscopic a-Al 50Ti 50 phase is entirely responsible for the hydrogen release during thermal desorption of milled NaAlH 4 + xTiCl 3. Isotopic H/D exchange has been observed by combined neutron and X-ray synchrotron diffraction on a PM NaAlD 4 + 0.04TiCl 3 sample, after exposing the milled sample to 20 bar H 2 at 50 °C for ca. 6 days. Under these pressure/temperature (P/T) conditions, disproportionation of NaAlD 4 is avoided, and ca. 32% of D atoms are exchanged with H atoms. Asymmetrically broadened reflections in the synchrotron data show peak splitting into two unit cell types, one expanded with H, the other remaining close to pure D based unit cell dimensions. The 2-phase model when fitted to the neutron data demonstrates that ca. 56% of D atoms in ca. 58% of all unit cells are exchanged with H, yielding a NaAl(H 0.56D 0.44) 4 composition for the expanded unit cells. HD scrambling (1 bar mixture of H 2 and D 2 at 23 °C) performed on desorbed H empty PM NaAlH 4 + 0.1TiCl 3 shows classic H 2 + D 2 ↔ 2HD equilibrium mixing, demonstrating that nanoscopic Ti containing Al 1− x Ti x surface embedded phases perform a H 2 dissociation/recombination function that unadulterated NaAlH 4 cannot.