Phase transformations of metal oxide stabilised CaO

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

“The phase composition of sorbents stabilized by reactive metal oxides can change with cycle number if (i) several mixed Ca-stabilizer phases exist, (ii) the transformation to the thermodynamically most favorable phase is slow compared to the process timescale (minutes) and (iii) the phase stability is affected by the temperature and atmosphere changes during calcination and carbonation.^9, 52 The formation of mixed phases between CaO and a reactive stabilizer has been studied extensively using ex-situ XRD by taking samples after the respective heat and gas treatment steps.^9, 12, 52 Yet, the dynamics (and intermediate phases) of the respective transformations can only be understood in full detail when utilizing in-situ XRD combined with further structure sensitive techniques.^12, 23 For example, Kim et al.²³ studied the phase transformations of Ca₃Al₂O₆-stabilized CaO during cyclic operation using a combination of in-situ XRD, ²⁷Al-NMR and EDX-TEM, see Figure 8 and 9. While the as-prepared (calcined) sorbent was composed of CaO, Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃ phases (Figure 8 c–e), in-situ XRD during cyclic operation revealed a complex phase evolution. In the first 10 cycles the fraction of Ca₃Al₂O₆ continuously decreased, ultimately disappearing, while the content of the Ca₁₂Al₁₄O₃₃ phase increased together with a slight increase in CaO. Between cycles 20 and 30, the phases evolved further, showing a decrease in the fraction of Ca₁₂Al₁₄O₃₃ and an increase in CaO, suggesting a segregation of an Al₂O₃ phase. However, since no Al₂O₃ phases could be detected by XRD, an element specific technique, i. e. ²⁷Al-NMR, was applied. Between cycle number 10 and 30, conventional²⁷ Al-NMR shows Al^IV coordination, in agreement with the expected Al coordination in Ca₃Al₂O₆ and Ca₁₂Al₁₄O₃₃. On the other hand, after 30 cycles dynamic nuclear polarization surface-enhanced (DNP-SENS) ²⁷Al-NMR revealed signatures due to an Al^VI coordination owing to α-Al₂O₃ (Figure 8c). However, according to the CaO-Al₂O₃ equilibrium phase diagram,⁸⁵ between 650–900 °C and with a Al : Ca ratio of 1 : 9, CaO and Ca₃Al₂O₆ are the thermodynamically stable phases while Al₂O₃ is not expected to be present under these conditions.”

“Figure 8 (a) In-situ XRD during cycling of Al₂O₃-stabilized CaO, (b) Rietveld refinement of the in-situ XRD data of (a) and (c) ²⁷Al-MAS-NMR and DNP-SENS NMR of Al₂O₃-stabilized CaO over cycling. Adapted with permission from Ref. [23]; copyright American Chemical Society, 2018. (d) Crystalline structure of mayenite (Ca₁₂Al₁₄O₃₃) and (e) crystalline structure of calcium aluminate (Ca₃Al₂O₆).”

“Figure 9 (a) HAADF micrographs and TEM-EDX mapping of Ca and Al of Ca₃Al₂O₆-stabilized CaO. Reprinted with permission from [23]; copyright American Chemical Society, 2018. (b) HAADF micrographs and TEM-EDX mapping of Ca and Mg of MgO-stabilized CaO. Reprinted with permission from Ref. [76]; copyright Wiley-VCH, 2016.”