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PRINCIPAL PUBLICATION AND AUTHORS

Deciphering the structural dynamics in molten salt promoted MgO-based CO2 sorbents and their role in the CO2 uptake, M. Rekhtina (a), M. Krödel (a), Y.-H. Wu (a), A. Kierzkowska (a), F. Donat (a), P.M. Abdala (a), C.R. Müller (a), Sci. Adv. 9(26), eadg5690 (2023); https:/doi.org/10.1126/sciadv.adg5690 (a) Laboratory of Energy Science and Engineering, Department of Mechanical and Process Engineering, ETH Zürich, Zürich (Switzerland)

REFERENCES

[1] A. Dal Pozzo et al., ACS Appl. Energy Mater. 2, 1295 (2019). [2] A.H. Bork et al., Proc. Natl. Acad. Sci. U.S.A. 118 (2021).

formation, which is represented by the rate constant and the induction period (Figure 110b-c). Key questions that arise are: what atomic scale features cause the initial deactivation of the sorbent? and what mechanism lies behind its reactivation? To answer these questions, the performance descriptors were contrasted with the structural changes occurring in the sorbent.

Examining the trend of the inverse of the size of the MgO crystallites, denoted as 1 over cycle number (as depicted in Figure 110d), a noteworthy 42% reduction between the first and third cycles was observed, with a pronounced decrease within the initial three cycles. This observation indicated that sorbent deactivation during the second and third cycles can be attributed to the sintering of MgO, resulting in fewer available nucleation sites, which are presumably MgO surface defects. Interestingly, the reactivation of the sorbent, which starts at the fourth cycle, correlated with the continuous increase of the content of Na2Mg(CO3)2 (Figure 110e). Hence, the reactivation of the sorbent is explained by the in-situ formation of Na2Mg(CO3)2 crystallites, which

act as effective nucleation seeds for MgCO3 formation. In combination with thermogravimetric analysis - mass spectrometry (TGA-MS) and Raman spectroscopy analyses, it was revealed that Na2Mg(CO3)2 crystallites form through the partial decomposition of NaNO3 (T ≥ 450°C), leading to the formation of [Na+ NO2 ] and [2 Na+ O2 ] ionic pairs, which remain dissolved in the melt and subsequently react with CO2 and MgO during the carbonation step (Figure 111).

The findings made in this study lend fundamental insight into the mechanism of CO2 capture of MgO- based sorbents promoted with molten nitrates and have important implications for their further improvement. This investigation demonstrates that effective MgO- based sorbents require promoters with the following functionalities: (i) to be able to dissolve the ionic pairs [Mg2+ O2 ] and [Mg2+ CO32 ], and (ii) to provide nucleation sites for MgCO3 formation. Furthermore, this work demonstrates that nucleation seeds can be formed from the promoter phase in situ through a judicious choice of the pretreatment conditions.

Fig. 111: Schematics of the transformations occurring in MgO-20NaNO3 during cycling.