Density functional theory study of interaction between H₂O/CO₂/CH₄ and oxygen-containing functional groups on coal surface

Using quantum chemical Density Functional Theory (DFT) simulations, the adsorption mechanisms of three common molecular species in coal mines (H₂O, CO₂, and CH₄) onto four prevalent oxygen-containing functional groups on coal surfaces were investigated from a microscopic perspective. The electrostatic potentials of the adsorbate molecules and the functional groups were evaluated, along with the adsorption distances, adsorption energies, and Mulliken charge transfer before and after adsorption. The results indicate that the order of maximum positive electrostatic potential for the functional groups is:-COOH>-OH>-C=O>-OCH₃. The order of maximum negative electrostatic potential is:-OH>-OCH₃>-COOH>-C=O. For the adsorbate molecules, the order of both maximum positive and negative electrostatic potentials is H₂O>CO₂>CH₄. Adsorption energy calculations reveal that the adsorption strength of the three molecules follows the trend H₂O>CO₂>CH₄. Specifically, the adsorption strength of H₂O on the various functional groups follows the order -COOH>-OCH₃> -OH>-C=O, whereas the adsorption strengths of CO₂ and CH₄ follow the order -OCH₃>-COOH>-OH>-C=O. Mulliken charge analysis demonstrates that oxygen atoms in the functional groups readily accept electrons. A greater amount of electron transfer from the adsorbate correlates with a more stable adsorption configuration. The stability order of adsorption is confirmed as H₂O>CO₂>CH₄.