Theoretical Study About Effects of H2O and Na+ on Adsorption of CO2 on Kaolinite Surfaces

The density functional theory(DFT) was used to investigate the adsorptions of carbon dioxide(CO₂) on kaolinite surfaces and the influences of Na⁺ and H₂O on the adsorption. Both cluster and periodic models of kaolinite were considered. The calculated results indicate that stable complexes can be formed between adsorbed CO₂ and the surfaces of kaolinite in the presence or absence of sodium cation and water molecule. The Al-O octahedral surface has a larger adsorption affinity for CO₂ than the Si-O tetrahedral surface of kaolinite because the hydroxyl groups of kaolinite Al-O surface present more activity than the basal O atoms of the Si-O tetrahedral surface in the inter-molecular interactions. The existence of exchangeable sodium cations exerts the significant effect on the adsorption of CO₂ with the dramatic increase of the adsorption energy, while the presence of water molecule decreases the adsorption strength insignificantly. The calculated Gibbs free energies of the adsorption reveal that the adsorptions of CO₂ on all the investigated kaolinite surfaces are feasible thermodynamically in the gas phase. Surface free energy was calculated to provide the predictions of the surface stability as a function of temperature.