Carbon Dioxide Removal by Chemically and Thermally Reduced Graphene-Based Adsorbents

The increase of atmospheric CO₂ concentration over past few decades has hazardous effect on environments. In this study, few layers (7–9) graphene adsorbents, prepared by chemical and thermal reduction techniques, were investigated for CO₂ adsorption. The study showed the effect of graphene structures on CO₂ removal at three different temperatures (0, 25, and 50 °C) and in pressure range of 0–5 bar. The different preparation methods induced difference in graphene structure in terms of no. of layers (7–9), interlayer spacing (0.351–0.381 nm), surface area (152–409 m²/g), average pore size (2.0–4.1 nm) and pore volume (0.48–1.63 cm³/g) as well as concentration of surface functional groups (10–20% oxygen content), which in turn affected the CO₂ uptake. The chemically reduced graphene with least layer separation and lowest oxygen content showed highest surface area (409 m²/g) and highest CO₂ uptake of 3.48 mmol/g. The thermally reduced graphene samples exhibited a fluffier structure with good layer separation, but smaller surface area and increased pore volume. At 0 °C and 5 bar, CO₂ adsorption capacity (mmol/g) of thermally reduced graphenes was in order of graphene (Ar) (1.21) < graphene (air) (1.54) < graphene (H₂) (2.0). The highest heat of adsorption of 22.5 kJ/mol was observed for graphene (hydrazine) at initial adsorption conditions.