CO2 adsorption on 2D nanosheet surfaces

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

“Chemical analysis by inductively coupled plasma (ICP) spectroscopy indicated that the Cs-type saponite contained ~0.04 mmol/g Cs (see c_cesium in Table 1). According to a recently developed analytical method using the data of elution test, ¹³³Cs magic-angle spinning (MAS) NMR, and radiocesium interception potential, the local molecular structures, as e.g., nanosheet surface, nanosheet edge, and oncoming hexagonal cavity, have been shown to act as Cs adsorption sites^18,19. In this analysis, 69% of the loaded Cs are found to physisorb on the surface of the 2D nanosheet, which amounts to a concentration of ~0.03 mmol/g. The carbon, hydrogen, and nitrogen (CHN) elemental analysis revealed that the C contents, c_carbon, in the Na- and Cs-type saponite samples after CO₂ loading are ~0.02 and ~0.18 wt.%, respectively (see Table 1). In light of the fact that the samples are isolated from the air during CO₂ loading, the C contents detected in the CHN analysis are solely associated with CO₂. Therefore, CO₂ molecules are sorbed to the Na- and Cs-type samples at concentrations of ~0.02 and ~0.15 mmol/g, respectively (see c_CO2 in Table 1). The concentration of CO₂ higher than that of Cs on the nanosheet surface implies the adsorption of several CO₂ molecules at the Cs cation sites, which will be discussed more in detail later.”

“Ps lifetime spectroscopy prior to CO₂-loading reveals two kinds of open spaces for both the Na- and Cs-type saponite samples. The similar sizes of small and large open spaces with R₁ ~ 3 Å and R₂ ~ 9 Å are obtained for the Na- and Cs-type samples (see Table 2). Our former studies revealed that the above two open spaces commonly observed for both the Na- and Cs-type saponite samples are caused by overlapped nanosheets: the small and large open spaces are the consequence of one- and two-nanosheet insertion into the interlayer spaces¹⁵. The relative intensity of large open space I₂ for the Cs-type sample is ~25%, much higher than that of the Na-type sample, though the intensities I₁ of small open spaces at 5% are similar to each other. The high intensity I₂ for the Cs-type sample indicating the large amount of 9 Å open space is resultant from insufficient self-assembly toward densification, which originates from interlayer Cs cations with low hydration degree^16,17.”

“Figure 2 shows the ¹³³Cs MAS NMR spectra obtained for the Cs-type saponite before (black curve) and after CO₂ loading (red curve), and their difference spectrum (blue curve). The dominant signal at ~−134 ppm observed for the sample prior to CO₂ loading correspond to the Cs⁺ interlayer cations physisorbed on the surfaces of the 2D nanosheets, whereas the additional broad signal at ~−15 ppm originates from Cs₂O compounds at nanosheet edges^18,19,20. The dominant peak arising from the Cs⁺ interlayer cations is slightly shifted and becomes broader upon CO₂ loading, which can be seen in the difference spectrum as well. This demonstrates that the CO₂ molecules adsorb at the Cs⁺ cation sites on the nanosheet surfaces at a concentration of ~0.03 mmol/g, as estimated above.”