Unveiling Differential Functional Group Responses in CO₂/CH₄/N₂ Adsorption/Desorption of Deep Ordos Coal via In Situ DRIFTS

The engineering of CO2/N2-enhanced coal bed methane (ECBM) has garnered considerable interest because of its economic and environmental advantages. Nevertheless, there is significant variation in how coal molecules adsorb CO2, CH4, and N2. Different from relevant molecular simulations and ex-situ experiments, this study employs in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to investigate the response behaviors of functional groups in anthracite during gas adsorption and desorption at varying pressures. The adsorption mechanisms of CO2, CH4, and N2 by various functional groups present in anthracite were determined. The results show that various functional group types can serve as key sites for the adsorption of CO2, and the order of the interaction intensity between the functional groups and CO2 is as follows: aromatic rings > coal macromolecule C–O > aromatic ring C–H > C = O ≈ aliphatic −CH3 > aliphatic −CH2– > aliphatic C–H. The electrostatic interaction between oxygen groups and N2 is favorable for N2 adsorption, while the aliphatic groups are unfavorable for N2 adsorption. The order of the interaction intensity between the functional groups and N2 is as follows: aromatic rings > coal macromolecular C–O > aromatic ring C–H > C=O. The aromatic rings in anthracite act as key sites for adsorbing CO2, CH4, and N2 gases, and the order of the adsorption capacity is as follows: CO2 > CH4 > N2. In contrast, the oxygen groups hinder CH4 adsorption. The findings of this study provide a theoretical foundation and practical guidance for CO2/N2-ECBM.