Effects of nitrogen and oxygen functional groups and pore width of activated carbon on carbon dioxide capture: Temperature dependence

In this work, activated carbon (AC) was prepared from bamboo waste using heat treatment. The AC was then modified to be nitrogen- and oxygen-enriched ACs by integration with urea, air oxidation, and KOH activation. At 25 degrees C, the nitrogen-enriched sample showed the highest CO2 adsorption affinity (uptake in pore at low pressures) and capacity (uptake in pore at moderate pressures, i.e., 1 bar). Whereas even at 0 degrees C, it still gave the highest affinity, but its capacity was reduced to be slightly lower than other samples. Consequently, a Grand Canonical Monte Carlo simulation was performed to macroscopically and microscopically investigate the CO2 adsorption behavior occurring in the experiments. The graphitic slit pore, in the pore width range of 0.7-1.5 nm, without a surface functional group (SFG), with pyridine (N-6), and with hydroxyl (OH) functional groups were modeled. (1) Adsorption affinity: the active site of SFG is dominant where CO2 molecules have the strongest interaction with N-functional group for all studied temperatures. The simulated results are consistent with the experimental data. (2) Adsorption capacity: the effect of pore width is more relevant. A sample with a more effective pore size gives higher capacity. However, the effective pore widths oscillated with temperature. High capacity at a suitable pore width resulted from the balance between the energy of motion and the packing of adsorbed molecules in order to optimize the energy. The energy of motion is more distinctive at high temperature; whereas, the commensurate packing is essential at low temperature.