Environmental Engineering Reference
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effects. Several MOF have shown promising results at 77 K with adsorption
capacities up to 7.3%, while at room temperature hydrogen uptakes is still not
higher than 1% [ 138 , 139 ].
Recent strategies for improving carbon nanostructures performance based on
the spillover mechanism have been investigated. Hydrogen spillover uses a sup-
ported metallic catalyst able to dissociate the hydrogen molecule. During the
successive phase of the process hydrogen atoms migrate to the substrate and finally
permeate through the substrate surfaces and/or into the bulk materials [ 140 ].
Thanks to this technique, an appreciable higher storage capability at room tem-
perature (up to about 2.4%) has been obtained by some authors for different types
of materials [ 141 ].
Chemical hydrogen storage could represent another interesting alternative
approach. It consists of on-board hydrogen generation through a chemical reaction
which starts from a hydrogen-containing material, characterized by high gravi-
metric energy density, followed by final off-board regeneration. The storage
material needs to be produced, and usually involves hydrolysis reaction. Specific
chemical hydrides are used, such as sodium borohydride (NaBH 4 )[ 142 ]. The
following chemical equation describes the process occurring in aqueous solution:
NaBH 4 þ 2H 2 O ! NaBO 2 þ 4H 2
ð 2 : 30 Þ
Equation 2.30 is irreversible, moderately exothermic, and can be controlled in
the presence of a suitable catalyst. When used on-board of vehicle the residual
compounds must be removed and regenerated off-board. This technique could
represent an applicative approach especially if liquids are used to store hydrogen,
but results too expensive and limited by very high regeneration energy require-
ments [ 143 ]. Recently, the Los Alamos laboratories have proposed a new process
able to improve the efficiency of the regeneration phase by addition of appropriate
digesting and reducing agents to the spent fuel [ 144 ].
References
1. Mcdowall W, Eames M (2006) Forecast, scenarios, visions, backcasts and roadmaps to the
hydrogen economy: a review of the hydrogen future literature. Energy Policy 34:1236-1250
2. Hetland J, Mulder G (2007) In search of a sustainable hydrogen economy: how a large-scale
transition
to
hydrogen
may
affect
the
primary
energy
demand
and
greenhouse
gas
emissions. Int J Hydrogen Energ 32:736-747
3. Ball M, Wietschel M (2009) The future of hydrogen—opportunities and challenges. Int J
Hydrogen Energ 34:615-627
4. Holladay JD, Hu J, King DL, Wang Y (2009) An overview of hydrogen production
technologies. Catal Today 139(4):244-260
5. Chorkendorff I, Niemantsverdriet JW (2003) Heterogeneous catalysis in practice: hydrogen.
In: Concepts of modern catalysis and kinetics. Wiley-VCH Verlag GmbH & Co, Weinheim
6. Rostrup-Nielsen T (2005) Manufacture of hydrogen. Catal Today 106:293-296
7. Armor JN (1999) The multiple role for catalysis in the production of H 2 . Appl Catal A
176:159-176
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