Theoretical calculation of potassium based sorbent carbon capture

https://doi.org/10.1016/j.ccst.2021.100011

“Electronic structural and phonon properties of K₂CO₃•1.5H₂O were studied by the density functional theory (DFT) and phonon lattice dynamics to better clarify the carbonation pathways of potassium-based adsorbents (Fig. 4). K₂CO₃•1.5H₂O showed a monoclinic structure with CO32− groups combined with H₂O to form a chain structure along the (010) facet (Duan et al., 2012). Thermodynamic calculations confirmed that the interaction between K₂CO₃ and CO₂ was much stronger than that of K₂CO₃•1.5H₂O under the given temperature range. Besides, the driving force for K₂CO₃ to capture CO₂ was greater than that of K₂CO₃•1.5H₂O. Three different regions for K₂CO₃•1.5H₂O, K₂CO₃ and KHCO₃ were observed in sequence on the phase diagram (Duan et al., 2012). This indicated that the transformation amongst the three phases depended on carbonation temperature and gas pressure. This had provided insights into the understanding of the carbonation pathways. The K₂CO₃•1.5H₂O intermediate could only capture CO₂ or being regenerated below the transition temperature, and carbonation or regeneration of K₂CO₃ occurred above the transition temperature. One prominent advantage of K₂CO₃•1.5H₂O was that it required less energy for regeneration than K₂CO₃. Thus, K₂CO₃•1.5H₂O might be suitable for capturing CO₂ from post-combustion flue gas under a temperature lower than its phase transition temperature and a steam pressure lower than 1.0 bar. K₂CO₃•1.5H₂O could also be applied for pre-combustion CO₂ capture when the steam pressure exceeded 1.0 bar (Duan et al., 2012; Duan and Sorescu, 2010; Duan et al., 2011).”

“Fig. 4. Structural and thermodynamic properties of K₂CO₃•1.5H₂O (a) crystal structure; (b) heat of reaction; (c) Gibbs free energy; (d) contour plots of calculated chemical potentials. Reproduced with permission from (Duan et al., 2012). Copyright 2012 American Chemical Society.”

“DFT methods have also been employed to investigate the co-adsorption of CO₂ and H₂O on different surface planes of K₂CO₃ to reveal its carbonation mechanisms. It is known that K₂CO₃ generally has two crystal structures as the monoclinic and hexagonal crystal geometries (Zhao et al., 2009c). Gao et al. studied the co-adsorption configurations for H₂O and CO₂ on the (001) surface of the monoclinic K₂CO₃, and they found that H₂O was adsorbed much stronger than CO₂ (Gao et al., 2013). This confirmed the formation of K₂CO₃•1.5H₂O from K₂CO₃ crystal was much easier than the formation of KHCO₃ through direct carbonation. This also indicated that the low coverage of CO₂ might limit the rate of bicarbonate formation (Gao et al., 2013). Liu et al. reported that the (001) surface of the monoclinic and hexagonal K₂CO₃ crystals exhibited different co-adsorption configurations for H₂O and CO₂. Calculation results indicated that K₂CO₃ surface showed substantially stronger affinity towards H₂O than CO₂ molecules. However, the hexagonal crystal K₂CO₃ had two (001) surfaces as the layer of potassium atoms ((001)-1) and the carbonate groups ((001)-2). Particularly, the (001)-1 surface of the hexagonal K₂CO₃ exhibited competitive capability towards H₂O and CO₂ adsorption. Two parallel paths for bicarbonate formation were then proposed, on the basis of the stable co-adsorption configurations for H₂O and CO₂. One-step mechanism was considered as that the OH group from H₂O interacted with the C atom from CO₂ to form bicarbonate, and this could well explain the carbonation pathways of the monoclinic and hexagonal K₂CO₃. Two-step mechanism involved the dissociation of H₂O to form OH and H, and the interaction between gaseous CO₂ and the dissociated OH to form bicarbonate. The carbonation path over the (010) surface of the hexagonal K₂CO₃ could be well explained by the two-step mechanism. Suggestions were then made to add proper supports or promoters to enable more (001)-1 surfaces in the hexagonal K₂CO₃ being exposed for enhanced bicarbonate conversion during CO₂ capture (Liu et al., 2017).”