Optimizing pore structure and edge surface functionalization with oxygen-containing groups in nanoporous graphene for highly efficient CO2 capture

Two-dimensional carbon-based materials with engineered porous structures and tailored functionalization are essential for enhancing CO₂ adsorption. Using density-functional theory calculations, we revealed that CO₂ adsorption in oxygen-functionalized nanoporous graphene depends on pore size, type of oxygen group, and group density. Epoxy groups enable moderate, stable physisorption with minimal distortion, while hydroxyl groups strongly enhance adsorption in medium pores (8.47 Å) with two groups through local bending and polar interactions. Carboxyl groups, however, induce strong chemisorption and distortions in small pores (3.80 Å), reducing stability, though more favorable binding appears in medium (8.47 Å), large (13.21 Å), and extra-large (18.18 Å) pores at lower densities. These behaviors are consistently reflected in adsorption energy, separation distance, and charge-transfer results. Electronic structure analysis further reveal that the stability of CO₂ adsorption is predominantly dictated by the coupled effects of orbital hybridization and charge-density localization. Epoxy, hydroxyl, and carboxyl functionalities contribute to CO₂ stabilization through different underlying mechanisms — namely electronic perturbation, polar orbital interaction, and chemisorption — each of which becomes more pronounced with increasing functional-group coverage. Electronic-structure analyses further demonstrate that small- and medium-sized pores provide the most effective spatial confinement for enhancing CO₂–functionalized surface interactions, whereas the larger pores yield comparatively weaker stabilization owing to reduced hybridization and limited electronic confinement. Among all systems examined, medium pores decorated with double hydroxyl groups provide the most favorable electronic environment, achieving strong local polar interactions and controlled hybridization without inducing excessive delocalization. These findings highlight that synergistic and precise tuning of pore architecture and functionalization is essential for optimizing nanoporous graphene as an efficient CO₂-capture material.