https://link.springer.com/article/10.1007/s11051-022-05518-z
“The CaO nanoparticles described above (samples 4–6) were used for absorption measurements. Samples 4–6 were exposed to air at different temperatures (25, 100, and 200 °C) for three weeks (= 504 h). The time dependences of the mass increase of the three CaO samples are shown in Fig. 7. Table 3 shows the initial and final masses of the samples kept at different temperatures. As follows from Fig. 7, the initial mass increase is much faster compared to the same at longer times. Moreover, after about 450 h, all samples reach saturation, i.e., their masses do not increase any more with further holding time. The half-time of curves in Fig. 7 decreases with increasing temperature, i.e., the relative rate of the process increases with temperature. More importantly, the relative mass increase of the samples decreases with increasing temperature.”
“Figure 7 The time dependence of the relative mass change of CaO samples 4–6 exposed to normal air at different temperatures”
“Table 3 The measured initial and final masses of CaO samples (data from Fig. 7)”
| Sample | Temperature, °C | Initial mass, g | Final mass, g | Final/initial masses | Half-time, h |
|---|---|---|---|---|---|
| 4 | 25 | 0.87 | 1.54 | 1.77 | 37 |
| 5 | 100 | 0.75 | 1.03 | 1.37 | 22 |
| 6 | 200 | 0.88 | 1.09 | 1.24 | 19 |
“Figure 8 The XRD patterns of the CaO nanoparticles (samples 4–6) after being kept in air at 25, 100, and 200 °C for 3 weeks”
“In contrary to the results found at room temperature, the same CaO sample also captured some moisture from air when it was kept in air at 100–200 °C during 3 weeks (see Fig. 8). These samples transformed into the mixture of two different phases: Ca(OH)2 and CaCO3. The calcite form of CaCO3 was formed at both temperatures, but at 100 °C, CaCO3 was found partly as aragonite. The sample kept for 3 weeks at 100 °C is composed of 74 w% of Ca(OH)2 and 26 w% of CaCO3; the latter divided into 17 w% of calcite and 9 w% of aragonite. The sample kept for 3 weeks at 200 °C is composed of 60 w% of Ca(OH)2 and 40 w% of CaCO3 in the form of calcite. This is in agreement with Fig. 2 and Table 2, where the mixture of (Ca(OH)2 + CaCO3) is reported at temperatures below 400 °C when Ca(OH)2 precipitate is calcined in air.”
“The fact that Ca(OH)2 is formed at 100 and 200 °C but not at 25 °C cannot be explained solely by thermodynamic reasons. It is probably due to kinetic reasons, namely to the lower activation energy of Ca(OH)2 formation compared to that of CaCO3 formation. In summary, with increasing temperature, thermodynamics prefers the formation of CaCO3, while kinetics prefers the formation of Ca(OH)2. The interplay between these two opposite trends might be the reason for the observed values. Certainly, further experiments are needed to clarify the temperature dependence in more details.”
“Finally, it is suggested here to perform capturing of carbon dioxide by CaO nanoparticles produced here at room temperature, as at higher temperature (at least up to 200 °C), the efficiency of this process is considerably decreased due to the partial formation of Ca(OH)2. Even if the rate of CO2 capturing is lower at room temperature, it does not require any industrial equipment: When CaO nano-particles are disposed on a field, they will capture the stoichiometric amount of carbon dioxide slowly, but surely.”