Effects of Surface Hydrophilicity and Heterogeneity on CO₂ Conversion to H₂CO₃ in the Nanopores of Layered Materials

CO₂ conversion to H₂CO₃ in water is an important reaction in many environmental and industrial processes, including CO₂ capture and storage. While this conversion reaction is well-studied in bulk water, it has been poorly investigated in confined water. Here we study the conversion reaction of CO₂ to H₂CO₃ in hydrophilic gibbsite and heterogeneous (hydrophilic/hydrophobic) Na-beidellite nanopores. We calculate the free energy of the reaction in the nanopores formed from these materials using ReaxFF metadynamics molecular simulations. We find that, regardless of the nature of the surfaces in these geological nanopores, both the activation free energy barrier and free energy of the reaction decrease compared to the reaction in the bulk environment. The activation free energy barrier is found to be higher in hydrophilic gibbsite and heterogeneous beidellite nanopores compared to the hydrophobic pyrophyllite nanopore. We observe the free energy of activation and reaction decrease with decreased pore size and surface hydrophobicity, favoring the formation of H₂CO₃. The water coordination structures obtained at reactant, product, and transition state basins support our free energy interpretations. The impeding factors in heterogeneous nanoconfinement for the formation of carbonic acid from carbon dioxide are interlayer cations and surface charge, whereas it is the surface hydroxyls for hydrophilic nanoconfinement.