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To investigate the adsorption mechanism of H 2 O, CO 2 , and CH 4 molecules on oxygen-containing functional groups (OFGs) in coal molecules, quantum chemical density functional theory (DFT) simulations were performed to study the partial density of states and Mulliken bond layout of H 2 O molecules bonded to different OFGs. The adsorption energy and Mulliken charge distribution of the H 2 O, CO 2 , and CH 4 molecules for each OFG were determined. The results showed that H 2 O molecules form 2, 1, 1, and 1 hydrogen bonds with −COOH, −OH, —C=O, and −O–R groups, respectively. Double hydrogen bonds connected the H 2 O molecules to −COOH with the smallest adsorption distances and highest Mulliken bond layout values, resulting in the strongest bonding between the H 2 O molecules and −COOH. The most stable configuration for the adsorption of these molecules by the −OH group was when the O–H bond in the OFG served as a hydrogen bond donor and the O atom in the H 2 O molecule served as a hydrogen bond acceptor. The order of the bonding strength between the OFGs and H 2 O molecules was Ph–COOH > Ph–OH > Ph—C=O > Ph–O–R. The adsorption energy calculation results showed that H 2 O molecules have a higher adsorption stability than CO 2 and CH 4 molecules. Compared with the −OH, —C=O, and −O–R groups, the −COOH group had a higher adsorption capacity for H 2 O, CO 2 , and CH 4 molecules. The adsorption stability of the CO 2 molecules for each OFG was higher than that of the CH 4 molecules. From the Mulliken charge layout, it was clear that after the adsorption of the H 2 O molecules onto the OFGs, the O atoms in the OFGs tend to gain electrons, while the H atoms involved in bonding with the H 2 O molecules tend to lose electrons. The formation of hydrogen bonds weakens the strength of the bonds in the H 2 O molecule and OFGs, and thus, the bond lengths were elongated.

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