78
7 Physisorption
isosteric heat and monolayer coverage value from existing thermodynamic statistics. Quantum mechanical studies were performed for the determination of adsorption energy on the inner and outer surfaces of the single-walled carbon nanotubes.
Lastly, force-matching and classical Lennard–Jones potential standard was used for
determining the values of specific surface area, monolayer coverage, and adsorption capacity. The physisorption of hydrogen was analyzed on zigzag single-walled
carbon nanotubes at temperature of −40 °C and 25 °C and pressures of 0.1 to 10 MPa.
All the data of the adsorption were explained via Toth model. The exterior physisorption energy of single-walled carbon nanotubes was in the range of 1.35–1.62 kcal/mol,
whereas the interior physisorption energy ranges from 1.22 to 2.43 kcal/mol at optimized diameter of 8–12 Å. At the optimized conditions and diameter, 1.75 wt. % of
hydrogen adsorption was obtained [20].
7.4 Physisorption of Hydrogen and Graphene
Graphene and graphene-based nanomaterials are actively involved in hydrogen
adsorption studies. It is understood that the large surface area; defect sites such as
carbon vacancy, wrinkles, and sheet edges; and presence of π-electron density out-ofplane of graphene sheet favor physisorption of incoming gas molecules. Therefore,
the hydrogen uptake capacity of graphene depends on how it has been processed.
Some of the studies that involve use of graphene for adsorption of hydrogen are given
in Table 7.1.
7.5 Physisorption of Hydrogen and Metallic Nanomaterials
Different metallic nanomaterials either in pure form or in the form of nanocomposites
are proving themselves useful in the area of hydrogen storage. In this section, different
metallic nanomaterials involved in the physisorption of hydrogen are discussed.
Metal organoclays are also used in hydrogen storage. A study has reported the
metal organoclays fabricated with integration of Boltorn polyol dendrimer H 30 (a
polyalcohol) in Na
+ -exchanged montmorillonite (NaMt). The polyalcohol addition
in the substrate clay was followed by in situ loading of Cu (0) and Pd (0) nanoparticles.
The polyalcohol-modified organoclays showed promising ability of CO 2 retention
(3.6–11.1 μmolg
−1 ) because of several OH
− groups of the added alcohol. However,
the integration of the metals in the clay drastically reduced the CO 2 retention but
caused a momentous increase in hydrogen uptake (51.8–508.2 μmolg
−1 ) where OH
−
groups served as the metal-stabilizing sites. The metal stabilization occurs because of
the increase in the formation of HO: Cu (0) and HO: Pd (0) associations and decrease
in HO: CO 2 interactions. The hydrogen and carbon dioxide retaining abilities of the
prepared nanomaterials were found to be closely associated with number of hydroxyl
groups of the incorporated polyalcohol moiety. The interactions of hydrogen were
7 Physisorption
isosteric heat and monolayer coverage value from existing thermodynamic statistics. Quantum mechanical studies were performed for the determination of adsorption energy on the inner and outer surfaces of the single-walled carbon nanotubes.
Lastly, force-matching and classical Lennard–Jones potential standard was used for
determining the values of specific surface area, monolayer coverage, and adsorption capacity. The physisorption of hydrogen was analyzed on zigzag single-walled
carbon nanotubes at temperature of −40 °C and 25 °C and pressures of 0.1 to 10 MPa.
All the data of the adsorption were explained via Toth model. The exterior physisorption energy of single-walled carbon nanotubes was in the range of 1.35–1.62 kcal/mol,
whereas the interior physisorption energy ranges from 1.22 to 2.43 kcal/mol at optimized diameter of 8–12 Å. At the optimized conditions and diameter, 1.75 wt. % of
hydrogen adsorption was obtained [20].
7.4 Physisorption of Hydrogen and Graphene
Graphene and graphene-based nanomaterials are actively involved in hydrogen
adsorption studies. It is understood that the large surface area; defect sites such as
carbon vacancy, wrinkles, and sheet edges; and presence of π-electron density out-ofplane of graphene sheet favor physisorption of incoming gas molecules. Therefore,
the hydrogen uptake capacity of graphene depends on how it has been processed.
Some of the studies that involve use of graphene for adsorption of hydrogen are given
in Table 7.1.
7.5 Physisorption of Hydrogen and Metallic Nanomaterials
Different metallic nanomaterials either in pure form or in the form of nanocomposites
are proving themselves useful in the area of hydrogen storage. In this section, different
metallic nanomaterials involved in the physisorption of hydrogen are discussed.
Metal organoclays are also used in hydrogen storage. A study has reported the
metal organoclays fabricated with integration of Boltorn polyol dendrimer H 30 (a
polyalcohol) in Na
+ -exchanged montmorillonite (NaMt). The polyalcohol addition
in the substrate clay was followed by in situ loading of Cu (0) and Pd (0) nanoparticles.
The polyalcohol-modified organoclays showed promising ability of CO 2 retention
(3.6–11.1 μmolg
−1 ) because of several OH
− groups of the added alcohol. However,
the integration of the metals in the clay drastically reduced the CO 2 retention but
caused a momentous increase in hydrogen uptake (51.8–508.2 μmolg
−1 ) where OH
−
groups served as the metal-stabilizing sites. The metal stabilization occurs because of
the increase in the formation of HO: Cu (0) and HO: Pd (0) associations and decrease
in HO: CO 2 interactions. The hydrogen and carbon dioxide retaining abilities of the
prepared nanomaterials were found to be closely associated with number of hydroxyl
groups of the incorporated polyalcohol moiety. The interactions of hydrogen were