# Synthesis and Characterization of Iron-Based Catalysts for Carbon Dioxide Valorization
## 3 Results
### Catalyst Characterization
All the peaks of the HZSM-5 diffractogram are attributed to the aluminum silicate phase of the zeolite (JDPDS Card No. 00-044-0003). Nonetheless, there are minor peaks assigned to Fe\({}_{3}\)O\({}_{4}\) and ZnFe\({}_{2}\)O\({}_{4}\) oxides for the corresponding samples. Figure 3: XRD diffractograms of supported samples. Figure 2: XRD patterns of unsupported Fe\({}_{3}\)O\({}_{4}\) nanoparticles. Figure 4 presents FTIR spectra for all unsupported samples. As the FTIR measurements are affected by humidity and atmospheric carbon dioxide, peaks detected within the region of 3100-3800 cm\({}^{-1}\) and 1600-1700 cm\({}^{-1}\) can be assigned to the O-H stretching and bending vibrations of the surface -OH groups [47; 48]. Additionally, the peaks at the band 1650-1830 cm\({}^{-1}\) correspond to the C=O bond stretching of the CO\({}_{2}\) molecule [48]. The effect of the promoters can be observed on the respective spectra. The broad band between 400 and 600 cm\({}^{-1}\) can be attributed to the Fe-O bond of magnetite [49; 50]. Specifically, the two intense peaks observed on the Fe\({}_{3}\)O\({}_{4}\) spectrum can be attributed to the Fe-O stretching vibration mode of the octahedral and tetrahedral sites, respectively [51]. The absorption band at 629 cm\({}^{-1}\), as well as the broad band at 640-730 cm\({}^{-1}\), can be linked with slight oxidation of the nanoparticles' surface (magnemic formation) [52]. The peak at 418 cm\({}^{-1}\) can also be associated with maghemite presence (Fe-O octahedral symmetric stretching) [53]. However, peaks at \(\sim\)420 cm\({}^{-1}\) and between 640 and 700 cm\({}^{-1}\) could also be attributed to akaganeite [54]. Peaks around 800 and 890 cm\({}^{-1}\) can be assigned to the \(\gamma\)-OH and \(\delta\)-OH stretching vibrations of goethite, respectively [55]. The peak at \(\sim\)1200 cm\({}^{-1}\) can be due to the -OH in plane deformation of the Fe-OH bond [56]. Spectra of Na- and K-doped samples present no significant variations, indicating that all the above species are possibly formed along with Fe\({}_{3}\)O\({}_{4}\). Nonetheless, the absorption bands of the ZnFe\({}_{2}\)O\({}_{4}\) spectrum are different; the substitution of Zn\({}^{2+}\) ions for Fe\({}^{2+}\) results in significant changes in the intensity of the peaks around 432, 623 and 693 cm\({}^{-1}\) due to disordering of the cation vacancies within the octahedral sites of the lattice, as ordering in these sites is highly affected by the foreign cation [57; 58]. Additionally, as zinc ions replace iron ions of the tetrahedral sites of the Fe\({}_{3}\)O\({}_{4}\) lattice, Fe-O vibration bands shift to lower frequency [59]. The Raman spectrum of the Fe\({}_{3}\)O\({}_{4}\) sample has four distinct peaks at 367, 508, 677, and 719 cm\({}^{-1}\) (Figure 5); 367 (T\({}_{2g}\)), 508 (Eg), and 719 (A\({}_{1g}\)) cm\({}^{-1}\) are characteristic of maghemite, while the additional peak at 677 cm\({}^{-1}\) can be attributed to magnetite [60]. The latter is the main peak (A\({}_{1g}\)), characteristic of magnetite. Peak deconvolution of the Fe\({}_{3}\)O\({}_{4}\) sample spectrum (Figure 6) supports identified phases of maghemite and magnetite. Also, the 3T\({}_{2g}\) active modes (\(\sim\)530, 440\(-\)480, and 190 cm\({}^{-1}\)), and E\({}_{g}\) (290\(-\)300 cm\({}^{-1}\)) of magnetite cannot be excluded as they are probably hidden within the observed broad bands [61]. The spectra of the doped samples present similar Raman bands (Figure 5). The Raman spectrum of cubic ZnFe\({}_{2}\)O\({}_{4}\) shows three characteristic peaks at \(\sim\)356 (F\({}_{2g}\)(2)), 492 (F\({}_{2g}\)(3)), and 665 (A\({}_{1g}\)) cm\({}^{-1}\)[62; 63]. The mode at 665 cm\({}^{-1}\) is related with the stretching of the Zn-O bonds in tetrahedrons [63]. The F\({}_{2g}\) modes are assigned to iron (F\({}_{2g}\)(2)) and zinc (F\({}_{2g}\)(3)) cation  
Figure 4: FTIR spectra of the unsupported samples. vibrations in octahedral sites [64].