Density functional theory approach to CO₂ adsorption on a spinel mineral: Determination of binding coordination

The mechanism of the adsorption of CO₂ onto various sites of MgAl₂O₄ (100), in particular with regards to binding coordination, was investigated using density functional theory (DFT) calculations. Of the available sites, CO₂ binding was calculated to be strongly adsorbed to oxygen atoms on the octahedral Al³⁺ and tetrahedral Mg²⁺ sites, with adsorption energy values of -1.60 eV and -1.86 eV, respectively, which was attributed to the small band gap of the CO₂-MgAl₂O₄ system. It is clearly found that strongly adsorbed CO₂ molecules bound to MgAl₂O₄ using polydentate (e.g., bidentate and tridentate) bonds. We also simulated the adsorption of multiple CO₂ molecules on MgAl₂O₄, and found three of eight CO₂ molecules to be strongly adsorbed using tridentate bonds onto the MgAl₂O₄ surface, with an interaction energy of -0.61 eV. The other five CO₂ molecules were also adsorbed, but weakly, i.e., using physical interactions with a modest binding energy of <0.10 eV and at a relatively long distance from the MgAl₂O₄ surface.