# Synthesis and Characterization of Iron-Based Catalysts for Carbon Dioxide Valorization
## 4 Discussion  
In the current study, iron-based catalysts, supported on HZSM-5 zeolite, have been synthesized for carbon dioxide valorization (Table 1). In this context, synthesized nanoparticles were doped with sodium (Na) and potassium (K) in five distinct chemical compositions at ratios of 1:1 and 1:2, and additionally a Zn-doped sample was also employed. All samples were subjected to comprehensive physicochemical characterization. Our aim was to correlate physicochemical properties with product distribution. Fe\({}_{3}\)O\({}_{4}\) is the primary phase detected for the reference sample, as well as for the alkali-doped materials (Figure 1). Nonetheless, apart from magnetite (Fe\({}_{3}\)O\({}_{4}\)), magnetite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)), akageneite (\(\beta\)-FeOOH), and geothetic (\(\alpha\)-FeOOH) are also detected (Figures 1, 4, and 5). The addition of alkalis, in different molar ratios, does not affect the formation of the above-mentioned phases, but slightly reduces the crystallite size and causes a small shift of the XRD peaks, as they have most likely been incorporated within the lattice. ZnFe\({}_{2}\)O\({}_{4}\) is the main phase detected for the Zn-doped sample; akageneite and maghemite are also present (Figures 1, 4 and 5). The addition of Zn slightly reduces crystallite size (Table 2). Due to the low nanoparticle loading on the supported samples, the main phases detected in all cases are magnetite and HZSM-5 (Figure 3). The main phases detected on the supported zinc-promoted sample are zinc ferrite and HZSM-5 (Figure 3). Analysis of high-resolution transmission electron microscopy (TEM) images reveal a slight increase in lattice plane from 2.94 to 2.98 A for the doped samples (Figure 9), along with a minor shift in the (220) X-ray diffraction (XRD) peak (Figure 1), suggesting the insertion of Na and K ions into the Fe\({}_{3}\)O\({}_{4}\) crystal lattice. The nanoparticles exhibit monocrystalline structures with primary particle sizes ranging from 5 to 20 nm, aggregating into larger agglomerates. Dynamic light scattering (DLS) measurements indicate that Na-Fe\({}_{3}\)O\({}_{4}\) (A) aggregates have a size distribution around 100 nm, while K-Fe\({}_{3}\)O\({}_{4}\) (A) aggregates are approximately 500 nm, and Fe\({}_{3}\)O\({}_{4}\) aggregates are to 900 nm (Figure 8). The addition of alkali ions in the Fe\({}_{3}\)O\({}_{4}\) lattice affects the charge of the nanoparticles. The size distribution measurement (DLS) of the nanoparticles takes place on suspension form, thus the higher the charge of the nanoparticle, the lower the aggregation. The catalytic behavior is assessed on the supported samples, where the nanoparticles are all dispersed in solid form on the surface of the zeolite micro-particle, thus all are aggregated in their solid form. TEM/SEM imaging on the hybrid Fe\({}_{3}\)O\({}_{4}\)/HZSM-5 catalyst confirms that these aggregates are dispersed on the HZSM-5 surface (Figures 10 and 11). The support provides the material with weak and relatively strong acidic sites, promoting bi-functionality, as catalysts contain metal sites from the iron nanoparticles and acid sites from HZSM-5 [74; 75]. The metal sites of the iron species are active in both RWGS and FTS reactions while the acid sites of the support promote oligomerization, isomerization, and aromatization reactions. NH\({}_{3}\)-TPD patterns of unsupported samples show that the unsupported iron-based oxides also contain acid sites, primarily of weak/medium and relatively strong acidity (Figure 7). As expected, basic sites are absent from the CO\({}_{2}\)-TPD patterns of all supported samples due to the low loading of nanoparticles on the acidic support (Figure S3). However, the unsupported samples present weak and medium basicity (Figure S4). In fact, the addition of K does not affect total measured basicity (Table S2) but causes shifts of the carbon dioxide desorption peaks towards higher temperatures, demonstrating the formation of stronger basic sites.