Environmental Engineering Reference
In-Depth Information
studied for their characteristics of high hydrogen content and low release tem-
peratures [
124
-
126
]. Reversibility and kinetic of the hydrogen absorption/
desorption cycle are critical issues also for this class of compounds. NaAlH
4
hydride can provide 5.5 wt% of hydrogen under reasonable conditions, by a
decomposition reaction made reversible thank to the usage of a catalyst [
127
],
while lithium alanate utilization appears not possible because of the irreversibility
of the first decomposition step.
Other solid materials are able to storage H
2
storage thanks to the adsorption
method. The process may be physical or chemical, in dependence of the energetics
of the adsorption mechanism. Physisorbed hydrogen is weakly bound to the solid
surface because of the 'attraction' between the adsorbate (hydrogen) and the
adsorbent (solid material); this is due to the induced dipole moment of a non-polar
adsorbate interacting with its own image charge in the polarizable solid. On the
other hand, chemisorbed hydrogen is strongly 'attached' to the material active sites
as result of a chemical bond formation between the adsorbent and the adsorbate in
a monolayer on the surface. Both the processes can occur especially on porous
materials as they require high surface area to maximize the available sites for
hydrogen uptake, while molecular affinity is a key parameter which determines the
selectivity of the material towards a particular molecule, especially for chemical
adsorption.
In the last decade high storage capacities have been claimed for carbon nano-
structures, theoretically able to match the automotive targets by means of
adsorption [
107
,
109
]. Activated carbon has been first proposed and has shown
promising storage capability of about half of liquid, at 50 bar, but exclusively at
77 K [
128
]. Other types of carbon materials based on nanostructured frameworks
have been then selected and proposed. In particular, the narrow pore size distri-
bution of single-walled carbon nanotubes (SWNTs) makes them attractive can-
didates as adsorbents for hydrogen molecules [
129
,
130
], but the early promising
experimental results have been the object of controversy [
131
,
132
].
On the other hand, also carbon nanofibers based on stacked graphite layers
seem show appreciable gravimetric storage capacity at room temperature and at
moderate pressure (about 120 bar) [
133
], but other experimental results obtained
on similar nano-materials appear contradictory [
134
], and the hydrogen storage
mechanism is still far to be understood.
Zeolites, which are a well-known class of molecular sieves and have been
proposed in the past for several industrial applications (ion exchange, adsorption,
heterogeneous catalysis), are also investigated, but their storage capacity remains
not satisfactory also at 77 K [
135
].
A recent scientific interest has been addressed towards a new class of structured
nanoporous materials, constituted by metal organic frameworks (MOFs), whose
potentialities are related to their low bulk density and very high specific area,
ranging from 1000 to 6000 m
2
/g [
136
]. They are synthesized by linking inorganic
clusters with various organic linkers [
137
], through strong bonds. The interest
towards MOF compounds is related to the theoretical possibility to optimize the
hydrogen storage exploiting both selective binding energy and high specific area
