Porosity of used MgO adsorbent

https://doi.org/10.1039/D2NA00213B

“The BET surface area, BJH adsorption pore volume, pore diameter, pore distribution, and density were obtained with MgO/CNT_Air and MgO to compare the structural properties (Table 2). The analyses were performed after the first carbonation–calcination reaction to examine the adjusted morphology after the initial reaction (Fig. 6(b) and (c)). The results indicate that MgO/CNT_Air had lower surface area and pore volume than those characteristics of MgO in the initial stage. In pore size distribution (Fig. S9†), larger pores were predominantly observed in MgO/CNT_Air than in MgO. The structural properties can be explained by the elongated plate-like structure of MgO/CNT_Air, in which the rigid structure reduced the surface area, pore volume, and small pores. Despite the reduced surface area and pore volume, the density of MgO/CNT_Air was lower than that of MgO. The empty spaces can enlarge the material’s volume and lower its density. Therefore, the obtained structural properties support the idea that the modified morphology of MgO/CNT_Air was an elongated plate-like structure with empty spaces.”

“The differences in structural properties indicate that MgO/CNT_Air had fewer active sites for CO₂ capture than MgO/CNT_Air in the initial cycle, which is consistent with the evaluation result of iterative CO₂ performance (Fig. 3). The structural properties after 30 cyclic reactions were obtained by performing cyclic reactions on BET samples using a tube furnace. The calcination reaction conditions were changed to 500 °C, 5 min under CO₂. These conditions were also employed when using TG-DSC, and the stable performance of MgO/CNT_Air and decreasing performance of MgO were re-confirmed (Fig. S10†). BET analysis results showed that the surface area and pore volume were reduced after 30 cycles. MgO exhibited a severe decrement in surface area and pore volume. The larger pores were also increased, which is evidence of sintering. MgO/CNT_Air also exhibited reduced surface area and pore volume, but the reduction was more moderate than MgO. The empty spaces might consume the large pores, and the average pore size became small. From the BET results, the modified morphology of MgO/CNT_Air effectively maintained the structural properties and delayed sintering.”