MgCO3 growth in NaNO3–MgO using EDX analysis and Raman micro-spectroscopy

https://doi.org/10.1039/D2TA02897B

“The location of MgCO₃ formed in the NaNO₃–MgO model systems after their exposure to CO₂ is identified by Raman micro-spectroscopy, SEM energy dispersive X-ray spectroscopy (EDX), and plasma FIB-SEM EDX. The Raman spectra of two locations and EDX maps of a mechanically cleaved cross-section of one of the grooves in the NaNO₃–MgO_A_5hours-CO2 sample are provided in Fig. 3. Fig. 3a illustrates the spots in which Raman spectra were collected (spot size of ca. 0.6–1.8 μm) and the area used for EDX analysis. In the illustration, the phases MgO, MgCO₃, NaNO₃ are highlighted in grey, blue and yellow colours, respectively, based on Raman spectra and EDX analysis. The Raman spectrum collected in a location in which NaNO₃ is present (colored yellow in Fig. 3a), shows bands at 101, 186, 724, 1067 and 1385 cm⁻¹ that are characteristic of NO₃⁻ in NaNO₃, Fig. 3b.^15,26–29 The most intense band at 1067 cm⁻¹ is attributed to symmetric stretching vibrations, while the bands at 101 cm⁻¹, 186 cm⁻¹, 724 cm⁻¹ and 1385 cm⁻¹ are due to asymmetric stretching vibrations of NO₃⁻.²⁹ A representative spectrum acquired close to the wall of the groove (blue coloured region in Fig. 3a) exhibit bands associated with NO₃⁻ and two additional bands that are due to MgCO₃.^30,31 These additional Raman bands at 740 cm⁻¹ and 1094 cm⁻¹ are assigned to bending and symmetric stretching vibrations of CO₃²⁻, respectively.^15,30,31 Bands due to NaNO₃ are observed in the spectrum acquired in the (blue) region that has been assigned to MgCO₃ because the size of the MgCO₃ particle (width ca. 1–2 μm) is of approximately the same size as the laser spot and within the lateral precision of the sample stage. Ex situ Raman micro-spectroscopy of samples that have been exposed to varying carbonation times showed no noticeable phase change of NaNO₃ with carbonation time, Fig. S4,† confirming the phase stability of NaNO₃ under the conditions investigated here. The region of the cross-section used for EDX analysis is shown in the overview SEM image in Fig. 3c and with a higher magnification in Fig. 3d. The respective EDX elemental maps are given in Fig. 3e–h. Comparing the magnified SEM image (Fig. 3d) with the elemental maps, it is clear that the structure observed at the wall of the groove contains Mg, C, O, while Na is absent, providing further evidence that the particle structure formed at the wall of the groove is indeed composed of MgCO₃, in agreement with Raman micro-spectroscopy analysis.”

“Fig. 3 EDX analysis and Raman micro-spectroscopy of a NaNO₃ filled groove (NaNO₃–MgO_A_5hours-CO2) after its exposure to CO₂ for 5 hours using a mechanically cleaved cross-section. (a) Illustration showing the location of the region used for EDX analysis and the two spots at which Raman spectra were acquired. (b) Raman spectra acquired at the two spots indicated in (a), i.e. inside the NaNO₃ promoter and at the wall of the groove where MgCO₃ was formed. (c) SEM image showing the entire cross-section including the NaNO₃ remaining at the bottom of the groove. (d–h) magnified SEM image and EDX elemental maps of Mg, Na, C, and O of the region highlighted in (a).”