Study on the Micro-mechanism of Controlling CH 4 and CO 2 Adsorption by Constructing Structural Defects in Coal Based on DFT

Structural defects extensively developed in tectonic coal are key factors influencing coalbed methane adsorption and CO2 geological sequestration. Existing research has insufficiently elucidated the microscopic mechanisms by which intrinsic structural defects in coal macromolecules regulate gas adsorption, and lacks systematic analysis of typical defect types. To elucidate the regulatory patterns and microscopic mechanisms of different structural defects in tectonic coal on CH4 and CO2 adsorption, periodic graphene models were constructed based on 7 × 7 aromatic layers, including single vacancy (SV), double vacancy (DV), multiple vacancies (MV1, MV2), Stone–Wales (SW) line defects, and nitrogen (G-N) and sulfur (G-S) substitution point defects. Employing density functional theory (DFT) coupled with the Dmol3 module, we systematically analyzed the bonding characteristics and formation energies of different defects. We investigated the adsorption behavior of CH4 and CO2 on perfect/defective aromatic layer surfaces from multiple dimensions, including electrostatic potential distribution, density of states, dipole moment, work function, charge transfer, orbital hybridization, and interaction energy. Results indicate: Formation energies of structural defects in tectonic coal exhibit significant differences, with vacancy-type defects possessing substantially higher formation energies than SW-type and doped defects. Defect introduction induces redistribution of bond lengths and angles in aromatic layers, increases electron density and electrostatic potential in defect regions, alters HOMO and LUMO energy levels, narrows the band gap, and significantly enhances the reactivity of coal aromatic layers. Single vacancy (SV) defects exhibited the largest positive and negative electrostatic potential values. The charge redistribution induced by defects was the core mechanism enhancing gas adsorption, with a more pronounced regulatory effect on the more polar CO2. The positive and negative electrostatic potentials for CO2 were 2.52 and 2.97 times those for CH4, respectively. Both CH4 and CO2 adsorption on all defect surfaces are physical in nature. The SV defect exhibits the highest charge transfer with both gases, while the SW defect possesses the largest dipole moment. Different defects show distinct adsorption capacities for CH4 and CO2, with the DV defect demonstrating the most pronounced adsorption enhancement for CH4 (interaction energy: −30.54 kJ/mol). while G-S defects exhibit the strongest CO2 adsorption (interaction energy: −40.70 kJ/mol). Overall, CO2 adsorption energies across all defect surfaces exceed those of CH4. Defects significantly enhance gas adsorption capacity in tectonic coal by regulating electron transfer efficiency and electrostatic interaction strength. This study reveals the microscopic mechanism by which structural defects in tectonic coal regulate CH4 and CO2 adsorption, providing crucial theoretical support for preventing coal and gas outbursts, efficient coalbed methane development, and CO2 geological sequestration.