Nanostructured CaO

https://doi.org/10.1002/cssc.202002078

“Since CaO-based sorbents derived from naturally occurring precursors such as limestone do not reach full CaO conversion within reasonable timescales and experience rapid deactivation with increasing cycle numbers, intensive efforts are undertaken to design more effective sorbent materials. An ideal CaO-based CO₂ sorbent displays a rapid and (close to) full conversion over many carbonation-calcination cycles.^{12, 14, 50, 51} Since a drop in the CO₂ uptake capacity is associated largely with pore blockage due to the large difference in the molar volume between CaO and CaCO₃ and diffusion limitations with increasing thicknesses of the carbonate layer, an ideal sorbent should contain a largely meso-porous (≤100 nm particle sizes) morphology.^{14, 18, 21, 39, 52, 53} To this end, template-assisted synthesis approaches have proven to be very useful.^{18, 24, 39, 53} For example, Wang et al.⁵⁴ used a sacrificial N-doped carbon nanosheet template to synthesize highly macroporous CaO nanosheets (CaN), see Figure 6a. Furthermore, Naeem et al.²⁴ developed hollow CaO microspheres utilizing spherical, carbonaceous templates (Figure 6b) and Kim et al.²⁶ obtained inverse opal-type CaO (Figure 6c) relying on carbon nanospheres as templates. These CaO-based materials show an enhanced CO₂ sorption capacity as compared to limestone, with CO₂ uptakes of up to 0.7 g /g_sorbent (Figure 6 d-e). Nevertheless, all three nanostructured materials suffered from sintering-induced deactivation (owing to the lack of a structural stabilizer), which led to a gradual destruction of the structured morphology (and it turn the CO₂ uptake) with cycling. Hence, nanostructuring alone is not sufficient to achieve fast and full conversion over many calcination-carbonation cycles.”

“Figure 6 SEM images of (a) CaO nanosheets, (b) CaO microspheres, (c) inverse opal-type CaO and (d) limestone. (e) CO₂ uptake of different CaO materials in the 1^st cycle and 10^th cycle in presence and absence of a stabilizer. (a) Adapted with permission from Ref. [54]; copyright Elsevier, 2019. (b) and (d) Adapted with permission from Ref. [24]; copyright Springer Nature, 2018. (c) Adapted with permission from Ref. [26]; copyright American Chemical Society, 2019.”