Page 323 - Adsorbents fundamentals and applications

308 SORBENTS FOR APPLICATIONS

in the past decade are mainly on sodium alanates (complex hydrides of sodium
and aluminum), the use of catalyst (such as Pd), and ball-milling the samples.
The thermodynamic equilibrium of the alanate hydride at 403 K is given in
Table 10.5. However, without adding a catalyst, the system cannot be rehydro-
genated. Bogdanovic and Schwickardi (1997) showed that by wet impregnation
of a Pd catalyst, the system could be made reversible, and the kinetics also
improved. Zidan et al. (1999) showed that “dry” addition of the catalyst was also
effective. With these improvements, the hydrogen storage capacity was still well
below the thermodynamic limits (Table 10.5).
Nano-crystalline metal particles have been developed more recently (Gleiter,
2000). The grain size is generally below 100 nm, and the range of 5–50 nm is
more typical. These materials have better properties for hydrogen storage than
their conventional counterparts.
Pd catalyst has also been used for other metal hydrides, such as Mg 2 Ni and
FeTi (Zaluska et al., 2001). Without the catalyst, an “activation” process is gener-
ally needed before hydrogen absorption, for example, FeTi needs to be activated
◦
by a series of long-term annealings at temperatures >400 C. Moreover, if the
material is subsequently exposed to air or water vapor, the activation process
has to be repeated. With the addition of Pd nanoparticles, the FeTi nanoparticles
could absorb hydrogen without activation.
The rates of absorption of hydrogen in all compounds listed in Table 10.5 are
slow, that is, in the order of tens of minutes for completion in nanoparticles.
For example, for absorption in nanoparticles of LaNi 5 at 298 K, 60 min for near
◦
completion is required. Approximately 100 min is necessary for Mg 2 Ni at 200 C
(Zaluska et al., 2001). Desorption is generally slower than absorption, and the
slow rates are detrimental to applications. The kinetics can be improved the use
of a catalyst. Figure 10.24 shows the effects of Pd on the hydrogen uptake rates
on LaNi 5 . It has been found that ball-milling the samples could reduce the grain
size of the crystals and consequently accelerates the rates for both absorption
and desorption by an order of magnitude (Zaluska et al., 2001). The effects of
ball-milling on the rates for Mg 2 Ni are shown in Figure 10.25. The smaller grain
size reduced the diffusion distance for H atoms. The effects of ball-milling have
been studied on a number of metal hydrides; however, they are not limited on
the grain size reduction alone. The effects of ball-milling are not understood,
although mechanochemistry is clearly involved. Titanium has been found to be
an effective catalyst for NaAlH 4 and Na 3 AlH 6 in both dehydriding and hydriding
kinetics, although at the expense of H-capacity (Sandrock et al., 2002). Ritter
and coworkers have shown evidence of TiAl alloy formation which is possibly
responsible for the catalytic effects (Riggleman et al., 2002).

10.3.2. Carbon Nanotubes
3
The U.S. department of Energy has set 6.5% (wt.) and 62 kg H 2 /m as the targets
for on-board hydrogen storage in fuel cell applications in vehicles (U.S. DOE,
1998; Hynek et al., 1997), at ambient temperature. The pressure is not specified,