DFT study of CO2 and H2O co-adsorption on carbon models of coal surface

The moisture content of coal affects the adsorption capacity of CO2 on the coal surface. Since the hydrogen bonds are formed between H2O and oxygen functional group, the H2O cluster more easily adsorbs on the coal micropore than CO2 molecule. The coal micropores are occupied by H2O molecules that cannot provide extra space for CO2 adsorption, which may leads to the reduction of CO2 adsorption capacity. However, without considering factors of micropore and oxygen functional groups, the co-adsorption mechanisms of CO2 and adsorbed H2O molecule are not clear. Density functional theory (DFT) calculations were performed to elucidate the effect of adsorbed H2O to CO2 adsorption. This study reports some typical coal-H2O center dot center dot center dot CO2 complexes, along with a detailed analysis of the geometry, energy, electrostatic potential (ESP), atoms in molecules (AIM), reduced density gradient (RDG), and energy decomposition analysis (EDA). The results show that H2O molecule can more stably adsorb on the aromatic ring surface than CO2 molecule, and the absolute values of local ESP maximum and minimum of H2O cluster are greater than CO2. AIM analysis shows a detailed interaction path and strength between atoms in CO2 and H2O, and RDG analysis shows that the interactions among CO2, H2O, and coal model belong to weak van der Waals force. EDA indicates that electrostatic and long-range dispersion terms play a primary role in the co-adsorption of CO2 and H2O. According to the DFT calculated results without considering micropore structure and functional group, it is shown that the adsorbed H2O can promote CO2 adsorption on the coal surface. These results demonstrate that the micropore factor plays a dominant role in affecting CO2 adsorption capacity, the attractive interaction of adsorbed H2O to CO2 makes little contribution.