Figure 9. A) Relationship between the isosteric heat of
CO2 adsorption at zero coverage and the amount of nGr in
the composites; B) relationship between the amount of H2adsorbed and the amount of nGr in the composites.
Figure 10 shows the H2 adsorption-desorption isotherms
measured at – 196°C on UiO-66 and its composites. Desorption was fully
reversible for all materials studied. Table 2 shows the hydrogen amounts
adsorbed at 100 kPa, which are similar to those reported in the
literature [15, 21, 22]. As in the case of CO2, the
amount of hydrogen adsorbed in composites was higher than in pure
UiO-66. However, unlike CO2, a clear increase in the
hydrogen uptake with the increase in the nGr content was found observed
(Figure 9B). On UiO-66-nGr6, 23 % more of H2 was
adsorbed than on UiO-66. Even though H2 is expected to
be adsorbed in the smallest pores, no trend between the amount adsorbed
and the porosity could be found taking into consideration the pore
texture analyzed by N2 adsorption. The molecular
diameter of N2 is 0.349 nm while that of
H2 is 0.296 nm, therefore H2 molecules
should have access to pore diameters that are not accessible by
N2 molecules [39, 40]. Moreover, the absolute
quadrupole moment of N2 is higher than that of
H2, 0.46 and 0.28 [41], respectively, and this could
also have an effect on the differences observed [40]. Thus, the
dependence on the amount of nGr added might be related to the specific
morphology of the composite and to the enhanced diffusion of
H2 at – 196°C to the UiO-66 units assembled in the
vertical pseudo-walls.