Page 329 - Adsorbents fundamentals and applications

314 SORBENTS FOR APPLICATIONS

result was refuted by Hirscher et al. (2001), who obtained a maximum hydrogen
capacity of 1.5 wt % H 2 equivalent by using D 2 at 0.08 MPa and room temper-
ature. This amount was attributed to hydride formation of Ti metal.
In the tests of Tibbetts et al. (2001), two samples of SWNTs were included:
one from MER Corporation and one from Tubes@Rice. The latter has been used
by a number of groups. The adsorption capacities at 296 K and 3.59 MPa were
0.028 wt % for the MER sample and 0.050% for the Rice sample. The MER
sample contained only about 10–15% SWNTs, while the Rice sample was quite
pure with only a small amount of soot. The samples were also subjected to
◦
pretreatment by exposure to H 2 at 3.6 MPa while heating to 500 C. Apparently
the pretreatment was done in a closed, static system and no time of exposure was
given. Tests similar to those of Tibbetts et al. were performed by Zuttel et al.
(2002), and similar results were obtained. Zhu et al. (2001) reported 3.5 wt %
hydrogen capacity for well-aligned SWNT bundles and 0.5 wt % capacity for
random-ordered SWNTs, both at 290 K and 10 MPa. The extra amount for the
well-aligned SWNTs was attributed to the inter-tube spaces, as discussed in detail
by Dresselhaus et al. (1999). The question concerning a possible leakage of the
system used by Zhu et al. (2000) and others was raised by Tibbetts et al. (2001).
Graphite Nanofibers. As defined by Baker and Rodriguez (Rodriguez et al.,
1995; Chambers et al., 1998), graphite nanofibers are fibrous carbon consisting
of graphite platelets that are arranged parallel, perpendicular, or at an angle with
respect to the fiber axis. The GNF is grown by catalytic decomposition, discussed
2
in detail in Chapter 9. The platelets could be 30–500 ˚ A in cross-sectional area.
There is little or no distinction between MWNTs and GNF when the platelets are
parallel to the fiber axis. When they are at an angle, the GNF is referred as having
a “herring bone” structure. It was on this type of GNF that the highest hydrogen
capacities were reported by Rodriguez and co-workers (Rodriguez and Baker,
1997; Chambers et al., 1998; Park et al., 1999). Their work will be described
briefly below.
The catalysts for their GNF growth were based on Ni-Cu, although some Fe
was also added in some cases. C 2 H 4 /CO/H 2 mixture was used as the hydrocarbon
◦
source. The GNF were grown at 500–700 C. The resulting fibers were partially
purified in “mineral acids.” There is no doubt that some catalyst remained. The
pretreatment before hydrogen adsorption was an important step. The samples
◦
were pretreated in an inert atmosphere (typically Ar) at 800–1100 C. A strong
dependence of the hydrogen storage capacity on the pretreatment temperature
◦
was reported (Park et al., 1999). Pretreatment at 1000 C yielded the best result,
about 35 wt % hydrogen uptake at 1600 psi and ambient temperature.
The sorption was envisioned as hydrogen entering the platelets. Thus, the
pretreatment was intended to remove the surface functionalities on the edges of
the graphite. The functionalities on carbon have been discussed in Chapter 5.
Upon the removal of the functionalities, hydrogen could enter the interlayer
spacing or be adsorbed on the edges.
The TPD of surface oxides from graphite has been studied extensively (see for
example, Tremblay et al., 1978; Chen et al., 1993). Typically, a strong desorption