Influence of Li, Na, K, Rb, Cs addition for CaO-based carbon capture

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

“In a study aiming to obtain a comprehensive description of the effect of different alkali metal salts on the CO₂ uptake of CaO, Reddy et al. probed the influence of different alkali metals and precursors (ACl, AOH, and A₂CO₃ with A=Li, Na, K, Rb, Cs) via the wet impregnation of CaO using loadings of 10 wt % and 20 wt %.⁸⁷ The authors studied the CO₂ uptake in the first carbonation cycle at temperatures between 50 °C and 750 °C, showing that the CO₂ uptake in the first carbonation cycle increases with the electropositivity and ionic radii of the alkali metal (Cs>Rb>K>Na>Li), as shown in Figure 12. The authors hypothesize that alkali metals influence the affinity of CaO towards acidic CO₂, resulting in turn in an improved CO₂ uptake compared to CaO, even at low temperatures (i. e., 50 °C). XPS analysis of Cs₂CO₃-promoted CaO indicates that CO₂ preferentially reacts with CsO₂, which forms during calcination. Yet, we currently lack a mechanistic explanation of how the electropositivity of the ion or the ionic radii may affect the affinity of the sorbent towards CO₂. Indeed, morphological parameters such as the pore size distribution, pore volume and BET surface area have not been investigated in this study except for Cs/CaO and the CaO benchmark. Therefore, the results might as well be related to varying effects of the different alkali metals and precursors on the morphological characteristics of the sorbents rather than a change in their affinity towards CO₂, as has been reported before.^70, 91 To conclude, a better understanding of the effect of alkali metals on the textural properties of CaO during cycling will be pivotal to elucidate the effect of alkali metals on the CO₂ uptake of CaO.”

“Figure 12 TGA measurements of CO₂ sorption on alkali-metal doped CaO at (a) 50 °C and (b) at 500 °C; (c) XPS spectra of sorbents before and after reaction with CO₂ and d) atomic ratios of Cs to Ca at the surface over the course of one carbonation-calcination cycle for Cs₂CO₃-promoted CaO. Adapted with permission from Ref. [87]; Copyright American Chemical Society, 2004.”