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.