First principles Molecular Orbital/Density Functional Theory was used in this work to model two clay minerals, Na-montmorillonite and kaolinite, and their interactions with CO2 molecules. Energy minimizations were performed with the M06-2X functional, well suited for non-bonding interactions, followed by determination of the infrared vibrational frequencies. The free energies of adsorption of the clay models, the adsorbates, and the adsorbate-substrate clusters were determined, and the difference provided results for ΔGads. The values obtained are contrasted with data derived from experimental adsorption isotherms. Selected input structures indicated that the presence of water molecules, modeled explicitly, lead to significant differences in ΔGads. The outcomes depended strongly on the clay model (montmorillonite or kaolinite), the adsorbate, and the initial adsorbate-substrate cluster. Small differences in the initial geometry did not produce much change in the adsorption of CO2 on kaolinite, which remained unfavorable, but significantly changed the interaction of this adsorbate with Na-montmorillonite with outcomes as different as physisorption and mineralization. The vibrational frequencies for adsorbed CO2 were compared to experimental data and to calculations in the literature using Molecular Dynamics simulations. Energy scans for adsorbate moving away from the surface were performed to determine the activation energy and the rate constant of adsorption, finding a strong dependence on the position and nature of the surface complex as well as the number and distribution of the water molecules with respect to the surface.

DFT calculations of structure and dynamics of CO2 adsorption on kaolinite and montmorillonite surfaces

253rd National Meeting (2017)

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