Figure 6. SEM images of pure UiO-66 (A) and UiO-66-nGr2 composite (B)
samples. The assembled wall of nanoparticles on the nGr surface is
marked in the bottom middle micrograph.
Figure 7 shows the CO2 adsorption isotherms measured on
the four materials at 0, 20 and 40°C. The amounts of CO2adsorbed at atmospheric pressure are compared in Table 2 and are in the
range of those reported in the literature [14-18]. Upon the
composite formation, a ~20 % increase in the amount of
CO2 adsorbed was found. The best performing sample is
UiO-66-nGr1, and we link it to its highest volume of pores smaller than
0.5 nm in which the adsorption forces for CO2 are the
strongest and where, at the low relative pressure, the density of
CO2 is the highest [38]. The amount of
CO2 adsorbed on UiO-66-nGr2 and UiO-66-nGr 6 is similar
to that on UiO-66-nGr1, which is in agreement with the similarities in
the volume of ultramicropores in the composite materials. The
CO2 uptake decreased with the increasing temperature,
from 3 mmol/g at 0°C to 1.2 mmol/g at 40°C for UiO-66-nGr1, in good
agreement with the exothermic nature of CO2 adsorption
on these materials.