Green Synthesis and Characterization of Nanographene–MnO Composite Nanoparticles for CO₂ Capture: Adsorption Performance, Isotherm Analysis, and Reusability

In this study, hybrid composite nanoparticles (MnO-NG/green) containing nanographene (NG) and manganese oxide (MnO) were produced by using an environmentally friendly synthesis approach, and their CO₂ adsorption performance was investigated in detail. The conventional method was used to synthesize MnO-NG composites, which were then compared with MnO-NG/green composites prepared via green synthesis using bioextract from the hemp plant. The composite nanoparticles were structurally characterized using various analytical methods, including FTIR, XRD, SEM, TEM, EDX, and BET analyses. The surface morphology of the composites obtained through green synthesis demonstrated a more homogeneous and regular distribution of MnO nanoparticles on the NG surface. BET analysis revealed that the specific surface area of the MnO-NG/green composites was 629 m²/g, with a mean pore diameter of 4.65 nm. CO₂ adsorption tests were conducted at 273 and 298 K under 1 bar, and it was determined that the MnO-NG/green composites achieved 5.81 and 4.94 mmol/g CO₂ uptake capacities, respectively. These values were significantly higher than those of NG (2.59–2.07 mmol/g) and conventional MnO-NG (4.43–3.74 mmol/g) composites. Analysis of the adsorption isotherm models indicated that the experimental data were better fitted by the Langmuir model. The calculated isosteric heat of adsorption (Q_st, 19.4–24.5 kJ/mol) suggests that the adsorption of CO₂ by MnO-NG/green composites predominantly occurs via physisorption. The MnO-NG/green composites demonstrated high structural stability and thermal resistance in five-cycle reusability tests, showing a reuse efficiency of 97%. This study clearly demonstrates that the production of MnO-NG/green composite nanoparticles via a green synthesis offers a promising approach.