Raman spectroscopy and morphologies of used MgO adsorbent

https://doi.org/10.1039/D2NA00213B

“The chemical natures of MgO-based sorbents were analyzed by Raman spectroscopy after 30 cycles of carbonation–calcination reactions (Fig. 5). The peak appearances of MgO and MgO/CNT_Air are identical throughout the multicycle reaction, as can be seen in Fig. 1. In MgO/CNT_N₂, the peaks at 714 cm⁻¹ and 1091 cm⁻¹, representing the ν₄ asymmetric bending modes of CO₃²⁻ and NO₃²⁻ and ν₁ symmetric stretching mode of NO₃²⁻ ions, disappeared, respectively. In Fig. S5,† thermal decomposition of MgO/CNT_N₂ was validated by TG-DSC, representing the loss of AMS. Therefore, it is speculated that the decomposition of AMS occurred at the early stage of the multicycle reaction in the MgO/CNT_N₂ sample, which resulted in the rapid performance decay.”

“Fig. 5 Raman spectra of MgO-based sorbents after 30 cycles of carbonation–calcination reactions.”

“SEM images were obtained to compare the evolution of the structure during the multicycle reaction of MgO/CNT_Air and MgO, with results shown in Fig. 6 and 7. Supplementary SEM images are also provided in Fig. S7 and S8.† Images in Fig. 6(a)–(c) show the alteration of MgO flakes into spherical particles. The carbonation into MgCO₃ induced a morphology of stacked rhombohedral plates with the trigonal crystal structure (Fig. 6(b)). The regeneration of MgCO₃ produced spherical particle-based MgO structures (Fig. 6(c)). Interestingly, the structure of regenerated MgO was affected by the pristine tubular structure. Each agglomeration of spherical MgO particles exhibited one-directional attachment, producing elongated plate-like structures. We assume that the wall of the tubular structure changed to an elongated plate-like structure, and the inner empty space of the tube produced the empty spaces. Elongated plate-like structures and empty spaces were repeatedly observed in the following iterative reactions. After 100 reaction cycles, the MgO/CNT_Air sample retained the morphology of one-directional attachment of MgO particles and empty spaces, even though dense agglomerates were produced. In comparison, the MgO sample exhibited a non-directional agglomeration of spherical MgO particles (Fig. 7 and S8†). These agglomerates expanded during iterative reactions.”

“Fig. 6 SEM images of MgO/CNT_Air: (a) pristine, (b) 1^st carbonation (MgCO₃), (c) 1^st calcination, (d) 12^th calcination, (e) 30^th calcination, and (f) 100^th calcination.”

“Fig. 7 SEM images of MgO: (a) pristine, (b) 1^st calcination, (c) 12^th calcination, and (d) 30^th calcination.”