## 3 Results and Discussion
### Sorbent Behavior in Carbonation/De-Carbonation Multi-Cycle Test  
The reversibility of carbonation/de-carbonation reactions is limited by the sintering process, which leads to a gradual decay in the sorbent's CO\({}_{2}\) uptake capacity [21]. However, the practical use of the sorbent depends on its reactivity and stability during multiple cycles. In this study, multiple carbonation/calcination cycles were performed in the TGA to investigate changes in the sorbent's activities upon cycling. TGA was used to investigate the decomposition temperature of CaCO\({}_{3}\) and thus determine the optimum decarbonation temperature. The flow rate of CO\({}_{2}\) was 50 mL/min for the carbonation reaction, while  
Figure 8: Nitrogen adsorption isotherm of Ca6 Mg2 Ce1 obtained by BET. Figure 7: Porosity verses BET surface area results for the preparedorbents before CO\({}_{2}\) uptake. 30 mL/min of N\({}_{2}\) was used for the decarbonation step at 700 \({}^{\circ}\)C. The figure shows that purging N\({}_{2}\) gas at a flow rate of 300 mL/min at 700 \({}^{\circ}\)C was sufficient to level out the CO\({}_{2}\) uptake curve and ensure the complete decomposition of CaCO\({}_{3}\) back to CaO. Therefore, carbonation and decarbonation reactions were carried out at 700 \({}^{\circ}\)C. The data obtained on CO\({}_{2}\) uptake capacities over successive carbonation/de-carbonation cycles are shown in Figure 9. It was noted that all samples exhibited an increase in CO\({}_{2}\) uptake capacity after the first cycle. Ca11 Mg2 Ce1 and CaO had a dramatic increase in CO\({}_{2}\) uptake capacity after the first cycle. Ca11 Mg2 Ce1 increased from 17 wt.% to 54 wt.%, whereas CaO increased from 14 wt.% to 53 wt.%. After one cycle, all the selected samples showed a noticeable decline in CO\({}_{2}\) uptake capacities but at different rates. After 45 cycles, the overall decay of the CO\({}_{2}\) uptake capacities from the capacity of the first cycle was calculated to be 25% for Ca6 Mg2 Ce1, 29% for Ca4 Mg2 Ce1 and 39% for CaO  
From Figure 9, it was concluded that the stability of CO\({}_{2}\) uptake capacity results from small grains and porous surfaces. These results indicate that the addition of MgO and CeO\({}_{2}\) helped achieve this stability. The addition of these inerts at certain percentages to CaO can effectively prevent the agglomeration and size growth of CaO during carbonation/decarbonation multi-cycles [22]. However, the CaO content affects the sorbent reactivity. A very high CaO content is not favorable due to the low content of inert material, whereas a low CaO content results in a reduced amount of active CaO and, accordingly, less CO\({}_{2}\) uptake capacity. Additionally, doping CaO with Ce and other metal oxides creates more oxygen vacancies in the sorbent to promote the diffusion of CO\({}_{2}\)[22; 25; 26]. The increase in CO\({}_{2}\) uptake capacities in all samples after the first cycle indicates a microstructural change following the initial decarbonation reaction. The CO\({}_{2}\) released during the decarbonation reaction and changes in surface particle sizes from CaCO\({}_{3}\) back to CaO may result in the formation of micro-cracks on the surface, which would facilitate CO\({}_{2}\) movement during subsequent carbonation steps and thus increase the CO\({}_{2}\) uptake after the first cycle. These results and explanations are similar to those obtained by [PERSON] et al. (2021), who referred to this phenomenon as self-deactivation [27]. They explained that during sorbent calcination (high-temperature thermal pretreatment), a hard skeleton was formed. In the next adsorption-desorption cycle, a soft skeleton was generated, which enhanced the sorbent carbonation rate, thereby gradually increasing CO\({}_{2}\) adsorption capacity during the initial cycles.