KOH-activated graphite nanofibers as CO₂adsorbents

Abstract

Porous carbons have attracted much attention for their novel application in gas storage. In this study, porous graphite nano-fiber (PGNFs)-based graphite nano fibers (GNFs) were prepared by KOH activation to act as adsorbents. The GNFs were activated with KOH by changing the GNF/KOH weight ratio from 0 through 5 at 900°C. The effects of the GNF/ KOH weight ratios on the pore structures were also addressed with scanning electron microscope and N 2 adsorption/desorption measurements. We found that the activated GNFs exhibited a gradual increase of CO 2 adsorption capacity at CK-3 and then decreased to CK-5, as determined by CO 2 adsorption isotherms. CK-3 had the narrowest micropore size distribution (0.6-0.78 nm) among the treated GNFs. Therefore, KOH activation was not only a significant method for developing a suitable pore-size distribution for gas adsorption, but also increased CO 2 adsorption capacity as well. The study indicated that the sample prepared with a weight ratio of '3' showed the best CO 2 adsorption capacity (70.8 mg/g) as determined by CO 2 adsorption isotherms at 298 K and 1 bar. Key words: activated graphite nanofibers, KOH treatment, CO 2 adsorptionThe rapid increase of the greenhouse gas carbon dioxide (CO 2 ) in the atmosphere is generally thought to be a major factor in climate change. These changes are increasingly viewed as a threat to both the global economy and the natural environment [1][2][3][4]. This increase in CO 2 is mainly attributed to human activities [5][6][7][8][9][10]. However, methane (bases: CH 4 50%-70%, CO 2 30%-40%) is one of reproducible biomass energy, which can obtain available energy sources, but also generate abundant greenhouse gas that goes against the purity of CH 4 [10]. It is essential to find an ideal material to work out the austere problem. Usually, CO 2 capture and storage is an efficient green technology used to reduce emissions by transporting and storing CO 2 in deep geologic formations. The capture methods can be classified as post-combustion, pre-combustion, and air separation followed by oxyfuel combustion [7]. Among the currently available technologies, post-combustion is the most easily applied technology. It involves use of absorption, adsorption, cryogenic distillation, membranes, and gas hydrates.Up to now, high-performance adsorbents have been widely used for CO 2 capture. Conventional solid adsorbents include zeolites [11], activated carbon [12], mesoporous silica [13], metal oxide-based adsorbents (e.g., MgO and CaO) [14], and metal-organic frameworks (MOFs) [15]. Nevertheless, in each case, there are defects in the adsorption process. Zeolites and mesoporous silica have the drawback of poor CO 2 capture performance. With zeolites, adsorption of moisture could affect the stability of the zeolite frameworks, and demands high regeneration temperature (>573 K). This results in huge energy consumption for CO 2 desorption. Meanwhile, although the adsorption capacity for CO 2 of activated carbon is high due to pore sizes ranging from micr...

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