Theoretical calculation of potassium based sorbent carbon capture

“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., 2012Duan and Sorescu, 2010Duan 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).”

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