Physisorption and chemisorption of CO2 using 2-D nanosheets clay

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

“Figure 3a shows the ¹³C MAS NMR spectra obtained for (I) Cs-type saponite ¹³C-enriched CO₂ (¹³CO₂) unloaded, (II) Na-type saponite ¹³CO₂ loaded, (III) Cs-type saponite ¹³CO₂ loaded, and (IV) Cs-type saponite ¹³CO₂ loaded and subsequently heat treated at 200 °C for 2 h in the N₂ atmosphere. Before loading ¹³CO₂, no signal is observed in the ¹³C MAS NMR spectrum of the unloaded Cs-type saponite. Similarly to that, no signal appears in the NMR spectrum of unloaded Na-type saponite (data not shown here). Upon ¹³CO₂ loading, an intense peak and broad hump arising from ¹³CO₂ adsorption appeared at around the chemical shifts of 125 and 170 ppm (see (II) in Fig. 3a). This together with the above result of ¹³³Cs MAS NMR indicates that CO₂ adsorption occurs at Na⁺ cation sites on the surface of the 2D nanosheet. The dominant and additional low-field signals become intense for the Cs-type saponite (see (III) in Fig. 3a), providing the important information that the Cs-type saponite captures CO₂ molecules more efficiently than that in the Na-type one. After heat treatment at 200 °C for 2 h, the dominant peak disappeared, whereas the broad hump at ~170 ppm remained (see (IV) in Fig. 2a), demonstrating that the intense and small signals are ascribed to physisorption (dominant) and chemisorption (secondary) on the nanosheet surfaces, respectively. It is reasonably inferred from the study of a ternary catalyst composed of copper, zinc oxide, and alumina²¹ that the broad signals at ~170 ppm originate from carbonate species, such as Cs₂CO₃.”

“It is of interest that both the physisorption and chemisorption for CO₂ molecules occur at the alkali metal cations located on the surface of the 2D nanosheet. It is most probable that the CO₂ physisorption on the nanosheet surface is caused by the quadrupole interaction between the alkali metal cations and CO₂ molecules. Molecular orbital calculation, on the one hand, predicts the resultant carbonate species Cs₂CO₃ from CO₂ chemisorption, in which the CO₂ molecule stabilizes by bridging with two alkali cations on the nanosheet surface (Fig. 3b). The adsorption energy for the optimized structure calculated with the SCIGRESS program (Fujitsu Ltd. Japan) is −97.8 kcal/mol, which is similar to that of CO₂ chemisorption onto the graphitic surface with two-bond conformation²². The concentrations of CO₂ physisorption, c_PhysCO2 and CO₂ chemisorption, c_ChemiCO2 for the Na- and Cs-type samples obtained from the total amount of CO₂ and the ratio of two peak intensities in ¹³C MAS NMR spectra are listed in Table 1. It is noted that ~13% of the loaded CO₂ is instantaneously chemisorbed as the carbonate species in both samples without employing strong acid solution at ambient pressure and temperature. As the concentration of Cs⁺ cations physisorbed on the surface of the 2D nanosheet is ~0.03 mmol/g (see Table 1), the cesium carbonate formed with two Cs⁺ cations on the nanosheet surface has the concentration of ~0.02 mmol/g. This is consistent with the concentration of CO₂ chemisorption, c_ChemiCO2 (see Table 1), signifying that Cs⁺ cations on the nanosheet surface dominantly take part in the CO₂ chemisorption. On the other hand, the concentration of CO₂ physisorption, c_PhysCO2 is higher than that of Cs⁺ cations on the nanosheet surface. Presumably, a few CO₂ molecules are physisorbed at Cs⁺ cations with the distance of several angstroms (see Fig. 3b).”