The effect of humidity on the enhanced CO₂ adsorption of amine-functionalized microporous activated carbon

Cost-effective and environmentally friendly sorbents were created for capturing CO₂ by incorporating monoethanolamine (MEA) and tetraethylenepentamine (TEPA) onto a microporous activated carbon (AC) material. The application of a KOH reagent enhanced the surface area and pore volume of the carbon material. The BET, SEM, EDX, and FTIR techniques were employed to analyze the structural and surface properties of the developed samples. Raw AC possessed the highest surface area and largest micropore volume equal to 786 m²/g and 0.33 cm³/g, respectively. The amine impregnation increased the nitrogen content of the carbon material, but it also significantly reduced the BET surface area and total pore volume, which are primarily responsible for physically adsorbing CO₂ towards ACs. The CO₂ adsorption performance of the raw and impregnated ACs was experimentally evaluated using thermogravimetric analysis (TGA) at different adsorption temperatures (25 and 50 °C) and CO₂ concentrations (10 and 90 vol.%). The findings demonstrated that the raw AC exhibited the highest capacity for CO₂ adsorption. Specifically, at a temperature of 25 °C and pressure of 1 bar (10 vol.% CO₂, N₂ balance), the raw AC achieved an uptake of 1.2 mmol/g, which was 60.3% and 79.3% higher compared to the CO₂ uptake of MEA-AC (0.5 mmol/g) and TEPA-AC (0.3 mmol/g), respectively. It is surprising that the combined uptake of CO₂ + H₂O increased by 12.5 and 23.4 wt.% (equivalent to 7 and 13 mmol/g) for MEA-AC and TEPA-AC, respectively, when humid flue gas was taken into account under the conditions of 50 °C, with 10 vol.% CO₂, 4 vol.% H₂O, and N₂ balance. These results indicate that the presence of H₂O facilitates the chemisorption of CO₂ by the novel and highly promising carbon-based sorbent prepared in this study, leading to an increased capacity for adsorbing CO₂ under water vapor containing flue gases.