Predicting the adsorption configurations of water clusters on –COOH and –OH using DFT calculations

The water in coal causes many negative effects on the processing and utilization of coal. Oxygen-containing functional groups (OFGs) are primary adsorption sites for water molecules on coal surfaces. However, little is known about the adsorption configuration of water clusters on OFGs. In this study, density functional theory (DFT) calculations were used to predict the adsorption configurations of water clusters on –COOH and –OH. The results indicated that water clusters tend to form quadrilateral and cubic shapes on both –COOH and –OH, with quadrilateral water clusters being more stable than cubic water clusters. The cubic water cluster is adsorbed on –COOH through two water molecules that directly form hydrogen bonds with –COOH. In contrast, the cubic water cluster is adsorbed on –OH through a water molecule at one of its vertices. The strong attraction of –COOH to water molecules introduces a minor defect in the cubic water cluster. Because of the synergistic effect of neighboring OFGs on the adsorption of water molecules, the average adsorption energy of water molecules is greater than that of water clusters on a single OFG. These results provide valuable microstructural insights into the adsorption of water on coal.