NiFh clay based CO2 at different temperatures

“It has previously been concluded that the interlayer CO2 may cause an irreversible adsorption in clay, i.e. even if a clay sample is not exposed to the CO2 gas, CO2 molecules remain in the interlayer space48,49. This means that once intercalated with CO2 the clay mineral will retain these molecules. However, a temperature change can affect the CO2 retention50,51 and this makes the process of intercalation and release truly reversible. We found that at a certain threshold temperature, the intensity decreases until the contribution to the scattered intensity from the clay mineral with intercalated CO2 is negligible. Simultaneously, the peak corresponding to the dehydrated LiFh and NaFh reappears (data not shown). The threshold temperature, at which the CO2 is desorbed from the interlayer space of the clays, is highly dependent on the type of interlayer cation used. For LiFh, this temperature is about 35°C, whereas for NaFh it is about −15°C (Figure 5). This is consistent with the difference in size between the smaller Li+ cation versus the larger Na+ cation. Li+ has a more concentrated charge distribution than Na+ and can thus polarize the CO2 molecule more, forming a stronger bond to it. Loring et al also give a description of the CO2 intercalation mechanism20. In the case of NiFh the release, like the intercalation, has more complex features, as shown in Figure 6.”



“Comparing the NiFh spectrum in Figure 1 with Figure 6 one can see that upon heating, the second CO2 peak merges with the first CO2 peak for NiFh. With increasing temperature, the intensity of the peak at the highest d-spacing value (about 1.31 nm) decreases and at 45°C it completely disappears while the lowest d-spacing value peak shifts to lower values and eventually contains all the (001) scattering. It appears that the final intercalation state is different from the original dehydrated state. This could suggest the formation of a complex CO2-Ni2+ structure within the interlayer space of the NiFh clay mineral, not present in the case of LiFh and NaFh. It is known that water intercalation experiments with NiFh can form a structure called Brucite (Ni[OH]2). Such a structure is formed in the cation exchange process from LiFh to NiFh60. It is possible that a Brucite-CO2 interaction could have an effect on the behavior. In addition this could occur due to the partially occupied d-orbitals of the Ni2+ ions, which allow multiple coordination geometries with CO2. These geometries can be possibly achieved by interactions of Ni d-orbitals with free oxygen orbitals present in polarized CO2 molecules.”

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