Phase transformations of metal oxide stabilised CaO

“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.952 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.91252 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.1223 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 AlIV 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 AlVI 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.”


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