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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.”

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“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, cPhysCO2 and CO2 chemisorption, cChemiCO2 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, cChemiCO2 (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, cPhysCO2 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).”

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