Effect of vacancy defects on electronic properties and wettability of coal surface

Abstract

The water wettability of coal is a major factor affecting its flotation performance, and thus it is imperative to explore the wetting mechanism. Previous studies on wettability mainly focused on the contact angle and interfacial tension, and there are only a few reports on the electronic properties that affect the adsorption behavior of H2O. In this study, the electronic properties of perfect and defective surfaces of coal, the adsorption behavior of H2O on the surface, and wettability were tested using molecular simulations. X–ray diffraction (XRD) analysis of anthracite was performed, and two–layered graphite was used as an anthracite model. The results show that the aromatic structure of anthracite is very similar to the crystal structure of graphite; the difference lies only in the crystallite size: anthracite crystallites are smaller than graphite crystallites. Therefore, the modified graphite structure can be used to simulate the local structure of coal. The bandgap of defective surfaces is lower, their electronic density of states near Fermi energy is greater, and their electronic activities are better than those of perfect surfaces. Defects decrease the energy gap of frontier orbitals (ΔELUMO–HOMO) between H2O and the surface, and strengthen the interaction between H2O and the surface·H2O adsorbs physically on the coal surface with two hydrogen atoms facing the surface. Defects increase the wettability of the coal surface, and the order of wettability improvement by defects is: double vacancy defect > single vacancy defect > Stone–Wales defect.