3.2. CO2 adsorption
The high surface area SBET and pore size distribution of the activated carbon materials are beneficial for effective adsorption/desorption of gas. Fig. 5 shows the carbon dioxide adsorption isotherm of the activated carbon samples examined at adsorption temperatures (0 C and 25 C at 1 bar) by the volumetric method. Interestingly, the activated carbon GSK1-700 exhibits a high CO2 capture capacity of 7.49 mmol g1 at (0 C) and 1 bar, as shownin Table 2. This carbon dioxide capturing is the highest amongst of all the activated carbon materials prepared and measured under the same conditions. GSK1-700 shows a higher CO2 uptake (7.49 mmol g1) than the previously reported while the highest capacities reported so far are 7.4 and 7.2 mmol g1 for AG-2-700 [20] and MOF CAU-1 [43] respectively (see Table S1 and Fig. S6).
Indeed, several studies have indicated that the presence of heteroatoms in carbonized-activated carbons can improve their performance for CO2 capture [44]. However, the improved performance
of N-doped carbons compared to the un-doped carbons of CO2 capture in many cases is usually marginal. Up to date, there has beenno clear evidence to show that the heteroatoms present in
carbonized-activated materials play a significant role in CO2 capture/adsorption.
It is also noteworthy that CO2 adsorption capacities are closely based on the cumulative micropore volume (PV) calculated for pores up to 0.5 nm (see Table 1). Therefore, this
superior performance can be ascribed to higher micropore volume [45], the amount of residual hydroxyl groups in the activated carbons and the ratio of non-carbon elements such as N and O
retained in the activated carbons (Figs. S7 and S8), that led to an increase in the chemical adsorption of CO2 [46]. The existence of oxygen and nitrogen is identified by measuring the ratio of elements for activated carbons as shown in Table 1.