Activated Carbon from Manchurian Walnut Shells for CO2 Adsorption: A Comparison of CO2 and KOH Activation Methods

Abstract

In this study, a two-stage approach was employed to produce porous carbons from Manchurian walnut shells (MNS). The shells were utilized as a cost-effective biomass precursor for activated carbon production. The first step involved the carbonization of the shells, followed by activation using either carbon dioxide or potassium hydroxide. As a result, two series of biochar samples were prepared by varying the duration of CO₂ activation and the KOH-to-precursor ratio. This approach enabled an investigation of the CO₂ capture potential of the MNS-derived activated carbons. The structural evolution of the biochars was characterized using N₂ physisorption, scanning and transmission electron microscopy. The results revealed that the original cellular structure was well-preserved after carbonization, while the activation step led to its significant transformation. Notably, the specific surface area, total pore volume, and micropore volume achieved after KOH activation were more than three times higher than those after CO₂ activation. Nevertheless, CO₂ activation is more economical and simpler than the chemical KOH method. The microporous structure of the samples changed with the KOH/carbonized MNS ratio. As the amount of KOH increased, all textural properties improved, except for the volume of narrow micropores (size <0.8 nm). The CO₂ adsorption capacities of all samples were measured at 298 K and 1 bar. The maximum CO₂ uptake in each series was 3.0 and 4.0 mmol/g, respectively. These values are competitive within the current state of CO₂ adsorbent development.