Development of tubular-shaped MgO using CNT as the template

https://doi.org/10.1039/D2NA00213B

“This study aims to enhance the durability of MgO-based sorbents in the reversible carbonation–calcination reaction by modifying the morphology to preserve active sites. For this, tubular-shaped MgO particles were synthesized to restrain grain overlapping by producing inner empty spaces. In order to produce tubular MgO particles, multi-walled carbon nanotubes (CNT) were used as a template for the precipitated MgCO₃. Two samples were prepared, one by removing and one by retaining the CNT template, and their reaction behaviors were compared.”

“The synthesis method of MgO precipitated on the CNT template was modified by referring to the literature.^22,23 As the alkali metal salt (AMS) promoter, a hybrid salt of nitrate and carbonate, 2(Li_0.44K_0.56)NO₃/(Na_0.5K_0.5)₂CO₃, was employed for carbonation enhancement.^16,17 Three types of MgO-based sorbents, MgO/CNT_N₂, MgO/CNT_Air, and MgO, were synthesized. MgO/CNT_N₂ was a MgO-based sorbent on a CNT template. MgO/CNT_Air was a MgO-based sorbent on a sacrificial CNT template that was thermally removed afterward. The control sample MgO was prepared without CNT but in the same manner. The samples are listed in Table 1. The ratio of AMS promoter to MgO was determined according to the referenced studies.^16,17”

“Precipitation on the CNT templates

In order to form tubular-shaped morphology, MgCO₃ particles were precipitated on the activated CNT. 40 mg of activated CNT was dispersed in 40 mL of deionized water by sonication. 1.382 g of K₂CO₃ was dissolved in 20 mL of deionized water, followed by mixing with CNT solution at 50 °C via 300 rpm magnetic stirring. MgCl₂ solution was prepared by dissolving 0.952 g of MgCl₂ in 40 mL of deionized water. MgCl₂ solution was added dropwise to the CNT/K₂CO₃ solution and reacted for 24 h at 50 °C via 300 rpm magnetic stirring. The obtained MgCO₃/CNT powder was filtered and washed with deionized water. Then it was dried in a convection oven at 60 °C for 12 h. The MgCO₃ control sample was precipitated using 60 mL of 1.382 g K₂CO₃ solution and 40 mL of 0.952 g MgCl₂ solution. MgCl₂ solution was added dropwise to the K₂CO₃ solution and reacted for 24 h at 50 °C via 300 rpm magnetic stirring. The obtained MgCO₃ powder was filtered and washed with deionized water. Then it was dried in a convection oven at 60 °C for 12 h.”

“Thermal treatment

The prepared MgCO₃ samples were thermally decomposed to MgO using a tube furnace. For MgO/CNT_N₂, the MgCO₃ in the MgCO₃/CNT powder was thermally decomposed to MgO at 450 °C for 2 h in a tube furnace purged with N₂ gas. For MgO/CNT_Air, the MgCO₃ and CNT in the MgCO₃/CNT powder were thermally decomposed to MgO at 450 °C for 2 h in a tube furnace purged with air. For the MgO control sample, the prepared MgCO₃ was thermally decomposed at 450 °C for 2 h in a tube furnace purged with N₂ gas. The resulting MgO samples were then gently ground with a mortar and pestle.

Salt impregnation

2(Li_0.44K_0.56)NO₃/(Na_0.5K_0.5)₂CO₃ was impregnated as an AMS additive into the prepared MgO samples. To 150 mL of methanol, 0.33 mmol of LiNO₃, 0.42 mmol of KNO₃, 0.1875 mmol of Na₂CO₃, and 0.1875 mmol of K₂CO₃ were added. The salt solution was sonicated for 2 h and then stirred with a magnetic bar at 300 rpm for 18 h. To the 50 mL of salt solution, 0.1 g of the prepared MgO/CNT or MgO was added. The solution was sonicated for 2 h and mixed via 300 rpm magnetic stirring for 18 h. Then solution was evaporated at 50 °C for 3 h. The obtained salt-impregnated MgO samples were gently ground with a mortar and pestle.”

“Fig. 2 SEM and TEM images of pristine MgO sorbents. SEM images of (a) MgO/CNT_N₂, (b) MgO/CNT_Air, and (c) MgO samples, and TEM images of (d) MgO/CNT_N₂, (e) MgO/CNT_Air, and (f) MgO. Dotted areas in red and in green indicate CNT with MgO flakes, and tubular-shaped MgO flakes without CNT, respectively.”