Mechanisms of 2-D nanosheet clay based CO2 capture

https://doi.org/10.1038/s42004-020-00346-5

“This unique adsorption nature is explained by the interior structure of the open nanospace characteristic for 2D materials. Saponite possess a 2:1 layered structure with 2D nanosheets consisting of tetrahedra and distorted octahedra. O and Si atoms are located at the vertices and central site of the tetrahedron, respectively. On the other hand, the O atoms or OH groups sit on the vertices of distorted octahedron, whereas a metallic Mg atom is located at the central site of octahedron (see the inset of Fig. 4). In the case of stevensite, the same family of saponite in aluminosilicate-type 2D materials, ca. 6.7% of the central Mg atoms in the octahedra are absent (see the inset of Fig. 4). An influence of Mg missing in the octahedron on the chemical bond is visible in the wavenumber region of 600–850 cm⁻¹ in the FT-IR spectrum of stevensite. The FT-IR spectrum for the stevensite exhibits intense and small absorption peaks at the wavenumbers of around 650 and 775 cm⁻¹, which can be ascribed to Si-O-Mg and Mg₃OH bending vibrations, respectively²⁴. The wavenumbers of the above bending vibration bands for the stevensite are lower than those of saponite, as the missing Mg atom causes lower frequencies in the both bending vibrations. It is noted here that the absorption peaks of Si-O-Mg and Mg₃OH bending vibrations are red-shifted for CO₂ loaded saponite as well implying atom missing in the octahedron (see Fig. 4). It is reasonably inferred that the above red-shift arise from the disappearance of O atoms from the vertices of the octahedra by the analogy of stevensite as illustrated in the inset of Fig. 4.”

“Based on the present findings of ¹³³Cs and ¹³C MAS NMR and FT-IR spectroscopy, we can draw the scenario of instantaneous ex situ mineral carbonation as schematically illustrated in Fig. 5. In saponite, the O atoms are weakly bound to the octahedron by ionic bonding, in contrast to the strong semi-covalent bonding of O and Si atoms in the tetrahedron. CO₂ gas molecules thus easily pick up the O atoms from the octahedra at the nanosheet edges in the interior of the above open spaces to be activated as the carbonate ions of CO₃²⁻ type (see right-hand side of Fig. 5). The CO₃²⁻ ions are then chemisorbed at the alkali metal cations on the surface of 2D nanosheets (see left-hand side of Fig. 5). The softness of the octahedra is also anticipated from the fact that the decomposition of octahedral sheets by mechanochemical milling proceeds prior to tetrahedral sheets²⁵.”