Figure 7. CO2 adsorption isotherms on UiO-66 composites at 0°C, 20°C and 40°C in the pressure range 0−120 kPa on (A) linear and (B) logarithmic scale.
Figure 8A shows the isosteric heats of CO2 adsorption, whose values are in the range of those reported in the literature [15, 16]. Even though there is no visible trend in the amount adsorbed with an increase in the amount of nGr, an increase in the heats of adsorption was observed with an increase in the nGr content (Figure 9A). The latter suggests that the addition of nGr not only enhanced the amount of CO2 adsorbed but also increased the strength of CO2 adsorption, which might be related to a denser array of carbon atoms in nGr. It is important to mention that the heats of adsorption show little variations with the surface coverage, suggesting the homogeneity of the adsorption centers occupied by CO2 in each sample at our experimental conditions. Thus, based on the amount of CO2 adsorbed only 0.050 cm3/g would be covered. Therefore, the volume occupied by CO2 represents only a small fraction of the total pore volume and, based on the N2 adsorption results, the pores that have very homogeneous sizes contribute to this volume in all the samples.