Modulating Acidic Oxygen-Containing Functional Groups on Coal-Based Activated Carbon to Enhanced Methane Adsorption and Diffusion

The key to achieving efficient enrichment of low-concentration coal mine gas via pressure swing adsorption lies in the coordinated modulation of the adsorbent's pore architecture and surface chemical properties. This study examines methane adsorption on coal-based activated carbon modified via preoxidation and controlled KOH activation. At an optimal carbon to alkali ratio of 1:4, the material developed a micropore- and small mesopore-dominated structure, comprising 91.26% of the total pore volume. Structural evolution involved selective hydroxyl removal and surface group reorganization, with a CH₂/CH₃ ratio of 5.95 indicating enhanced physisorption potential. Oxygen-containing groups such as carboxyl and carbonyl contributed to localized electrostatic interactions, improving methane affinity. The adsorption process followed a two-stage diffusion mechanism: boundary layer transfer (k_L = 1.02 × 10^-4 m/s) and micropore diffusion (D_eP = 2.55 × 10^-12 m²/s). Above the critical surface coverage θ₀, diffusion resistance sharply increased, reducing diffusivity by 91%. These results highlight the importance of synergistic pore and surface modulation in optimizing adsorbent performance for methane capture and separation.