Density Functional Simulation Study of Surface Wettability of Coal Molecules with Different Degrees of Defects

To explore the adsorption mechanism of H₂O molecules on the surfaces of defective coal molecules and perfect bituminous coal molecules, the energy band structure, electronic density of states, electrostatic potential, and front orbitals on the surfaces of three coal molecule models were investigated using quantum chemical density functional theory (DFT) simulations. The adsorption energy and Mulliken charge layout of H₂O molecules with the surfaces of defective coal molecules and perfect bituminous coal molecules were similarly investigated. The results of the DFT calculations showed that the widths of the forbidden bands of the defective coal molecular surfaces were narrower, and the electrostatic potential values were smaller. In addition, they each had an increased conduction band near the Fermi energy level, a larger electronic density of states near the Fermi energy level, and a higher electron activity and electron density than those of the perfect bituminous coal molecular surface. While stable adsorption of H₂O molecules occurred on the surfaces of the single-vacancy-defective coal molecules, double-vacancy-defective coal molecules, and perfect bituminous coal molecules, the adsorption energy values were -39.401, -30.002, and -29.844 kJ/mol for the more stable configurations, corresponding to -0.022, -0.013, and -0.011 electrons gained by H₂O molecules, respectively. Wettability improved with the appearance of defects, and the order of improvement was single-vacancy-defective coal molecule > double-vacancy-defective coal molecule > no-defect coal molecule.