Effect of the Position of Amine Groups on the CO₂, CH₄, and H₂ Adsorption Performance of Graphene Nanoflakes

CO₂ separation from CO₂/CH₄ and CO₂/H₂ mixtures is essential in H₂ production from biogas. Although amine-functionalized carbon materials are excellent candidates for selective CO₂ adsorption, their adsorption performance depends on the amine edge position, which requires further clarification. Herein, 19 graphene nanoflakes (GNFs) bearing two amines at various positions on the zigzag, corner, and armchair edges were evaluated. All structures exhibited better CO₂ adsorption compared to CH₄ and H₂. In the GNFs with isolated amines, only one main bond (H₂N-CO₂) was involved in CO₂ adsorption, which was influenced by charge transfer from the GNFs to CO₂. GNFs with adjacent amines had an additional hydrogen bond (H₂N-CO₂) that became stronger with decreasing steric hindrance. The CO₂ adsorption performance of amines decreased as the number of methyl groups on the nitrogen increased, which interfered with CO₂ adsorption. The presence of H₂O hindered the interaction of amines and CO₂ owing to the strong hydrogen bond between H₂O and amines.