# Synthesis and Characterization of Iron-Based Catalysts for Carbon Dioxide Valorization
## 3 Results
### Catalyst Characterization
Peaks at 176 and 310 cm\({}^{-1}\) are indicative of the presence of akaganite [65]; nevertheless, the observed peak at \(\sim\)176 cm\({}^{-1}\) can also be attributed to the F\({}_{26}\)(1) mode of zinc ferrite [66]. Finally, the 728 cm\({}^{-1}\) corresponds to maghemite [67]. For all samples, the broad Raman peaks are linked to nano-particle size [62]. Catalyst acidity is considered a key property, especially for the hydrocarbon formation step of the reaction. In order to qualitatively and quantitatively determine the acidic sites on the catalytic surface, temperature-programmed desorption of ammonia (NH\({}_{3}\)-TPD) has been employed for all supported samples upon reduction. Acid sites that present desorption peaks at 150-250, 250-350, 350-500, and \(>\)500 \({}^{\circ}\)C are defined as weak, medium, strong, and very strong, respectively [68; 69]. Figure 7 presents the ammonia desorption  
Figure 5: Raman spectra of the unsupported samples. Figure 6: Raman deconvolution of unsupported Fe\({}_{3}\)O\({}_{4}\). peaks for all samples, including those of the HZSM-5 for reference. All desorption profiles follow the pattern of the support, which shows two desorption peaks at 200-250 \({}^{\circ}\)C and 400-500 \({}^{\circ}\)C that correspond to weak/medium and strong acid sites [70]. The addition of Fe\({}_{3}\)O\({}_{4}\) on the HZSM-5 does not significantly affect the temperature of the desorption peaks. The total acidity of the sample Fe\({}_{3}\)O\({}_{4}\)/HZSM-5, as calculated by the desorbed ammonia of the TPD profile, is slightly lower than that of the support (Table 3). Comparing the alkali-doped samples with the Fe\({}_{3}\)O\({}_{4}\)/HZSM-5 sample, the addition of K (K-Fe\({}_{3}\)O\({}_{4}\) (B)) results in small shifts on both temperature peaks towards lower values, while the addition of Na (Na-Fe\({}_{3}\)O\({}_{4}\) (A)) also causes a slighter shift. Despite the above, the acid site strength, as well as the total acidity, of all samples is similar. NH\({}_{3}\)-TPD measurements have also been performed for unsupported samples, showing relative acidity (Figure S2). Ammonia desorption peaks can be attributed to weak/medium acid sites, while the desorbed ammonia is not affected by the addition of alkalis (Table S1). Temperature-programmed desorption of carbon dioxide (CO\({}_{2}\)-TPD) has been employed for all supported and unsupported samples, in order to determine their basicity. Figure S3 presents the CO\({}_{2}\) desorption patterns for all supported samples, including that of the support. Desorption profiles do not show any significant peaks, indicating that the catalytic surfaces do not contain any basic sites. On the other hand, CO\({}_{2}\) desorption peaks of unsupported samples confirm the presence of basic sites (weak and medium basicity) on the catalyst surface (Figure S4). Desorbed amounts of CO\({}_{2}\) show that the employed dopants do not affect total basicity, but slightly modify the relative strength of the basic sites (Table S2). \begin{table}
\begin{tabular}{c c} \hline \hline
**Materials** & **Desorbed Ammonia (mmol NH\({}_{3}\)/g\({}_{\text{cat}}\))** \\ \hline Fe\({}_{3}\)O\({}_{4}\)/HZSM-5 & 0.637 \\ Na-Fe\({}_{3}\)O\({}_{4}\) (A)/HZSM-5 & 0.684 \\ K-Fe\({}_{3}\)O\({}_{4}\) (A)/HZSM-5 & 0.573 \\ Na-Fe\({}_{3}\)O\({}_{4}\) (B)/HZSM-5 & 0.637 \\ K-Fe\({}_{3}\)O\({}_{4}\) (B)/HZSM-5 & 0.633 \\ ZnFe\({}_{2}\)O\({}_{4}\)/HZSM-5 & 0.595 \\ HZSM-5 & 0.718 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Desorbed ammonia (NH\({}_{3}\)-TPD). Figure 7: NH\({}_{3}\)-TPD profiles of supported samples. Dynamic light scattering (DLS) measurements of Fe\({}_{3}\)O\({}_{4}\), Na-Fe\({}_{3}\)O\({}_{4}\) (A), and K-Fe\({}_{3}\)O\({}_{4}\) (A) are presented in Figure 8, along with the measurement for HZSM-5 (by Mastersizer).