## 3 Results and Discussion
### Sorbent Characterization  
#### 3.3.1 X-ray Diffraction (XRD)  
XRD patterns of the selected sorbents were collected to examine the phase changes in the sorbents before and after CO\({}_{2}\) uptake, as shown in Figures 2 and 3, respectively. All five fresh samples before CO\({}_{2}\) uptake showed similar profiles with three or four peaks corresponding to CaO, MgO, CeO\({}_{2}\) and Ca(OH)\({}_{2}\), regardless of the weight percentage of the precursor. The appearance of the Ca(OH)\({}_{2}\) peak may have been due to moisture contamination of the sample. However, Ca(OH)\({}_{2}\) decomposed to CaO at 450-500 \({}^{\circ}\)C [14]. Therefore, the presence of Ca(OH)\({}_{2}\) in the fresh sample may not have affected the sorbent's reactivity in the carbonation/decarbonation reactions at 700 \({}^{\circ}\)C and in the ethanol sorption-enhanced processes, which were performed at a temperature range of 550-700 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **CO\({}_{2}\) Uptake in Weight/\%** & **CO\({}_{2}\) Uptake in/Molar (*) \%** \\ \hline Ca4 Mg2 Ce1 & 25 & 66 \\ Ca6 Mg2 Ce1 & 29 & 66 \\ Ca3 Mg2 Ce1 & 10 & 33 \\ Ca2 Mg2 Ce1 & 16 & 42 \\ Ca1 Mg2 Ce1 & 7 & 69 \\ Ca7 Mg2 Ce1 & 6 & 12 \\ Ca11 Mg2 Ce1 & 17 & 30 \\ CaO & 14 & 18 \\ \hline \hline \end{tabular} (*) mole CO\({}_{2}\)/mole CaO.  
\end{table} 
Table 2: CO\({}_{2}\) uptake capacities for all the prepared sorbents in weight and molar percentage bases. Figure 3: XRD patterns of the sorbents after CO\({}_{2}\) uptake. (**A**) Ca6 Mg2 Ce1 (**B**) Ca4 Mg2 Ce1 (**C**) CaO (**D**) Ca11 Mg2 Ce1 (**E**) Ca1 Mg2 Ce1. Figure 2: XRD patterns of the sorbents before CO\({}_{2}\) uptake. (**A**) Ca6 Mg2 Ce1 (**B**) Ca4 Mg2 Ce1 (**C**) CaO (**D**) Ca11 Mg2 Ce1 and (**E**) Ca1 Mg2 Ce1. Examination of the CaO d-spacing in all the selected samples indicates that diffraction lines of CaO did not change significantly upon the addition of MgO and CeO\({}_{2}\) to the CaO-based sorbents in the different weight percentage as CaO peak observed at 37.5\({}^{\circ}\) had d-spacing of (d = 2.40731 nm, d = 2.40633 nm, d = 2.40545 nm, d = 2.40688 nm, d = 2.40427 nm) in Ca4 Mg2 Ce1, Ca6 Mg2 Ce1, CaO, Ca11 Mg2 Ce1 and Ca1 Mg2 Ce1, respectively, corresponding to the (200) plane with a d-spacing of 2.40587 nm. After CO\({}_{2}\) uptake, as shown in Figure 3, three samples yielded patterns of CaCO\({}_{3}\). Although CeO\({}_{2}\) and MgO could be used for CO\({}_{2}\) sorption, their presence in this study was as inert composites. This study investigates the possibility of using CaO doped with MgO and CeO\({}_{2}\) as CO\({}_{2}\) sorbents to be employed in the ESR process. The experimental investigations of ESR were conducted at temperatures above 550 \({}^{\circ}\)C using iron oxide as the catalyst (because the iron oxide reducing temperature was 534 \({}^{\circ}\)C). In this temperature range, MgO and CeO\({}_{2}\) maintained their inert nature as they did not participate in the carbonation reactions. The absence of magnesium carbonate (MgCO\({}_{3}\)) in the samples after CO\({}_{2}\) uptake confirmed the presence of MgO as a structural support since MgO did not reacts with CO\({}_{2}\) at temperatures over 400 \({}^{\circ}\)C [15]. On the other hand, the CeO\({}_{2}\)-based adsorbent exhibited CO\({}_{2}\) desorption peak temperatures near 390 K (116.85 \({}^{\circ}\)C) [19] and thus, CO\({}_{2}\) desorption occurs at a relatively lower temperature than in this study. No appearance of CeO\({}_{3}\) was detected in the XRD samples after CO\({}_{2}\) uptake. Nevertheless, employing CeO\({}_{2}\) and MgO as CO\({}_{2}\) adsorbents has potential for lower-temperature steam reforming processes and could be investigated in further studies. It was also noticed that the CaO phase was detected in all samples after carbonation, except for Ca1 Mg2 Ce1. This may be due to the low CaO content in this sample, which is only 13% (see Table 3). The Scherrer equation was used to calculate the crystallite size of CaO, Ca(OH)\({}_{2}\), MgO and CeO\({}_{2}\).