Environmental Engineering Reference
In-Depth Information
Figure 9.8 Diagrams [118] showing hydrogen
storage molecules. (a) Structures based on
ethylene C
2
H
4
; (b) indicates a single Ti atom
with bond length 2.04 A
from carbon, followed
by adsorption of two Ti atoms at binding energy
1.47 eV; (c) shows how this structure can be
regarded as a covalent bond between the lowest
unoccupied molecular orbital of the ethylene
molecule (large lobes in (d) ) with the 3D orbital
of Ti (small lobes in (d)). Right-hand side of the
figure shows binding sites of 10 hydrogen
molecules that are provided by the two titanium
atoms in this structure. This corresponds to 14
wt % of hydrogen.
tank, but might
flow out as the hydrogen fuel is withdrawn. The authors suggest that
the molecules should be embedded in a nanoporous matrix. The expected means to
bring hydrogen gas in or out of the storage sites is to adjust the temperature. A
nanoporous matrix might facilitate this, if it were continuous and could be uniformly
heated by passing current through it.
It has been recently shown [118b] that the adsorption sites for H
2
shown in
Figure 9.8 are blocked by even small amounts of O
2
. These authors say that to make
use of such designs, in practice, where some oxygen will always be present, access to
the surface by oxygen would have to be blocked by a polymeric coating or by a
nanoporous structure that would not allow molecular oxygen to enter. Palladium
-
silver alloys are permeable to hydrogen but not other gases, and could, in principle,
block entry of oxygen to a cask containing the activated surfaces as shown in
Figure 9.8. The idea of a porous structure that will allow a particular chemical
element to enter and leave is suggested in Figure 10.9. This
figure applies to Li ions,
but a similar picture might apply to H
2
.
Similar results for hydrogen storage in titanium-decorated polymers, notably poly-
acetylene (C
2
H
2
)
n
, have been presented by Lee et al. [119]. (Whether oxygen would
block these sites is not clear.) Polyacetylene forms sheets, so that containing the
material in the gas tank while hydrogen is drawn in and out is easier. The authors
say that titanium-decorated polyacetylene will store 9% by weight of hydrogen if
hydrogen is admitted at 25
C and initial pressure 30 atm, and is desorbed at 2 atm
and 130
C. These are practical conditions. The bonding geometries are similar to
those shown in Figure 9.8.
