A structural review of nanoscopic Al1−xTMx phase formation in the TMCln enhanced NaAlH4 system

Hydrogen absorption kinetics and structural features of NaAlH4 enhanced with transition-metal and Ti-based nanoparticles

“…We observe very broad reflections from AlPd, with a unit cell dimension a ¼ 3.0218 (1) A. As discussed for the H cycled PM NaAlH 4 þ 0.1PdCl 2 case in [23], we have not found concentration dependent lattice parameters for the wide homogeneity range reported in the phase diagram for bcc Al 1Àx Pd x (44À57 at.% Pd [34]), and as such we have modeled a 1:1 composition, which accounts for ca. 96.3% of the originally added Pd atoms.…”

“…All NaAlH 4 unit cell dimensions for our H cycled PM NaAlH 4 þ 0.1TM samples display normal dimensions, similar to their TMCl n enhanced NaAlH 4 analogues [23]. Considering that we have observed amorphous Al 1Àx TM x phases for H cycled TiCl 3 , VCl 3 and FeCl 3 enhanced NaAlH 4 [23], it is plausible that the remaining unaccounted for TM nanoparticles may have reacted to form a minor component of amorphous Al 1Àx TM x phase which is not easily discernible in diffraction data. The case for Ti (65 nm) discussed below in Section 3.2 highlights this point, where we observe a minor component of amorphous Al 1Àx Ti x in H cycled PM NaAlH 4 þ 0.1Ti (65 nm) directly by high resolution TEM, which is not easily discerned in high resolution diffraction data.…”

“…We note also that commercially available nanoparticles may also contain significant quantities of metal-oxides (either amorphous or crystalline), which may hinder the reactivity of the metal [35]. All NaAlH 4 unit cell dimensions for our H cycled PM NaAlH 4 þ 0.1TM samples display normal dimensions, similar to their TMCl n enhanced NaAlH 4 analogues [23]. Considering that we have observed amorphous Al 1Àx TM x phases for H cycled TiCl 3 , VCl 3 and FeCl 3 enhanced NaAlH 4 [23], it is plausible that the remaining unaccounted for TM nanoparticles may have reacted to form a minor component of amorphous Al 1Àx TM x phase which is not easily discernible in diffraction data.…”

“…Our previous studies demonstrate that Al 1Àx Ti x phases that form on the NaAlH 4 surface are typically 4e25 nm [3,20e22], and similarly, Al 1Àx TM x phases (where TM is a transition metal) for all other period 1 and 2 TMCl n enhanced NaAlH 4 samples [23] are < 50 nm. Attempts to comill Al 3 Ti with NaAlH 4 have produced similarly poor kinetic enhancement due to the use of micron sized Al 3 Ti, discussed extensively in [11].…”

Nanoconfinement degradation in NaAlH4/CMK-1

“…However, the NaAlH 4 system has been investigated thoroughly over the last >20 years and an extensive array of catalytic phases can allow low temperature (120e140 C) hydrogen release and uptake [10]. Many of these transition metal catalysts are added to the NaAlH 4 system as halide salts, that are reduced to metal-Al surface-alloys that are catalytically active, with the generation of an inactive sodium halide salt by-product [11]. Although the list of catalytic phases for NaAlH 4 is extensive, recent focus on CeCl 3 [12] or Ti-based nanoparticles [13] show great promise, even when compared to the well-studied TiCl 3 doped NaAlH 4 system [10].…”