# Synthesis and Characterization of Iron-Based Catalysts for Carbon Dioxide Valorization
## 3 Results  
### Catalyst Characterization  
Following material synthesis, characterization of supported and unsupported iron-based nanoparticles was performed employing selected methods (i.e., BET, XRD, FT-IR, Raman, DLS, SEM/TEM, NH\({}_{3}\)-TPD, CO\({}_{2}\)-TPD). Table 2 summarizes BET surface area and porous characteristics of supported and unsupported materials, whereas letters (A) and (B) signify the addition of alkalis to magnetite nanoparticles with a molar ratio of 1:1 (A) and 1:2 (B). HZSM-5 exhibits the highest BET surface area and pore volume, equal to 409.4 m\({}^{2}\)/g and 0.243 cm\({}^{3}\)/g, respectively, while surface areas and porous characteristics of the supported samples slightly decrease, probably due to the metal oxide loading on the support [41]. BET surface areas of all unsupported nanoparticles are similar. The addition of zinc slightly improves the surface area as a result  
of the synthesis procedure. As expected, the BET values of the supported materials are close to that of the support due to the low metal oxide loading. The addition of alkalis has no significant impact on the surface area of the catalyst and its porous characteristics. On the other hand, the pore volume and size of the ZnFe\({}_{2}\)O\({}_{4}\)/HZSM-5 material, supported on zeolite, are slightly decreased. Figure 1 shows the X-ray diffractograms of all unsupported samples. The main peaks identified for the reference sample (i.e., Fe\({}_{3}\)O\({}_{4}\)) are attributed to magnetite (JDPDS Card No. 01-071-6336), while the presence of a group of considerably weaker peaks can be assigned to the \(\beta\)-FeOOH (JDPDS Card No. 00-034-1266) phase (akaganeite). The observed 20 peaks correspond to (220), (311), (400), (422), (511), and (440) crystal planes of the Fe\({}_{3}\)O\({}_{4}\) cubic structure [42; 43; 44] (Figure 2). The presence of additional XRD peaks could be indicative of other iron structures, such as maghemite or non-cubic magnetite [45]. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Materials**} & **BET Surface** & **Pore Volume** & \multirow{2}{*}{**Pore Size (nm)**} & **Crystallite Size** \\  & **Area (m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** & & **(nm) * \\ \hline Fe\({}_{3}\)O\({}_{4}\) & 145.6 & - & - & 11.52 \\ Na-Fe\({}_{3}\)O\({}_{4}\) (A) & 145.1 & - & - & 10.61 \\ K-Fe\({}_{3}\)O\({}_{4}\) (A) & 148.3 & - & - & 10.34 \\ Na-Fe\({}_{3}\)O\({}_{4}\) (B) & 136.1 & - & - & 10.90 \\ K-Fe\({}_{3}\)O\({}_{4}\) (B) & 151.3 & - & - & 11.21 \\ ZnFe\({}_{2}\)O\({}_{4}\) & 178.3 & - & - & 9.16 \\ Fe\({}_{3}\)O\({}_{4}\)/HZSM-5 & 398.2 & 0.210 & 7.3 & - \\ Na-Fe\({}_{3}\)O\({}_{4}\) & 401.1 & 0.194 & 6.6 & - \\ K-Fe\({}_{3}\)O\({}_{4}\) (A)/HZSM-5 & 395.8 & 0.194 & 6.7 & - \\ Na-Fe\({}_{3}\)O\({}_{4}\) & 387.8 & 0.173 & 6.2 & - \\ (B)/HZSM-5 & 388.4 & 0.182 & 6.8 & - \\ ZnFe\({}_{2}\)O\({}_{4}\)/HZSM-5 & 389.5 & 0.148 & 4.5 & - \\ HZSM-5 & 409.4 & 0.243 & 8.6 & - \\ \hline \end{tabular} \({}^{*}\)Results of the XRD analysis, using the Scherrer equation.  
\end{table} 
Table 2: BET surface area and porous characteristics. Figure 1: XRD diffractograms of unsupported samples. The diffractograms of all unsupported samples doped with Na or K show that the above-mentioned phases are also present (Figure 1). The calculated crystallite size of samples doped with Na or K is lower compared to the reference material, while among the doped materials, crystallite size does not differ noticeably (Table 2). However, small shifts of the main Fe\({}_{3}\)O\({}_{4}\) diffraction peak for the doped samples have been detected, suggesting that the dopant ions have possibly been incorporated into the crystal lattice [46]. Peaks attributed primarily to ZnFe\({}_{2}\)O\({}_{4}\) (JDPDS Card No. 00-001-1108) have been identified on the ZnFe\({}_{2}\)O\({}_{4}\) sample, along with smaller peaks of \(\beta\)-FeOOH. The diffractograms of all supported samples are presented in Figure 3. Due to the low iron oxide loading, for all samples, the dominant peaks are those of the HZSM-5 support.