CO2 Adsorption Behaviors of Biomass-Based Activated Carbons Prepared by a Microwave/Steam Activation Technique for Molecular Sieve

In this study, the activated carbon was prepared with superior CO2 selective adsorption properties using walnut shells, a biomass waste, as a precursor. The activations were conducted at various times using the microwave heating technique in a steam atmosphere. The surface morphology and chemical composition of activated carbon were analyzed using a scanning electron microscope and energy-dispersive X-ray spectroscopy. The textural properties were investigated using the N2/77K isothermal method, and the structural characteristics were examined using X-ray diffraction analysis. The CO2 and H2 adsorption properties of activated carbon were analyzed using a thermogravimetric analyzer and a high-pressure isothermal adsorption apparatus, respectively, under atmospheric and high-pressure conditions. Depending on the activation time, the specific surface area and total pore volume of the activated carbon were 570-690 m2/g and 0.26-0.34 cm3/g, respectively. The adsorption behaviors of CO2 of the activated carbon were different under atmospheric and high-pressure conditions. At atmospheric pressure, a significant dependence on micropores with diameters less than 0.8 nm was observed, whereas, at high pressure, the micropores and mesopores in the range of 1.6-2.4 nm exhibited a significant dependence. However, H2 adsorption did not occur at relatively low pressures. Consequently, the prepared activated carbon exhibited superior selective adsorption properties for CO2.