# Synthesis and Characterization of Iron-Based Catalysts for Carbon Dioxide Valorization
Conventional CO\({}_{2}\) hydrogenation can proceed via two distinct reaction pathways; via the RWGS/FTS or the methanol-mediated route, whereas in both cases product distribution is highly affected by the selected catalytic system and/or the reaction conditions [16; 17]. The first route involves the conversion of CO\({}_{2}\) into carbon monoxide (CO) through the reverse water-gas shift (RWGS) reaction, followed by Fischer-Tropsch synthesis (FTS) to generate C\({}_{1+}\) hydrocarbons. The second route, methanol to hydrocarbons (MTH), involves an initial CO\({}_{2}\) transformation into methanol, which is subsequently converted into C\({}_{1+}\) hydrocarbons. Direct RWGS/FTS CO\({}_{2}\) hydrogenation that can take place in a single reactor process is preferable with respect to economic aspects. Nonetheless, this approach mainly results in the production of CO and light paraffins, due to low CO hydrogenation and extensive olefin hydrogenation activity [18]. Conversion of CO\({}_{2}\) into C\({}_{1+}\) hydrocarbons requires the presence of active catalytic phases so that CO\({}_{2}\) is adsorbed and activated, thereby forming carboxylate species [16]. The latter, due to hydrogen presence, form -HOCO intermediates that break down to -OH and -CO, which are hydrogenated into -H\({}_{2}\)O, CO, and -HCO/-CH\({}_{\times}\), producing paraffins or olefins via subsequent hydrogenation/dissociation/dehydration steps [19]. Thus, catalyst selection must focus on bifunctional activity on both reactions, under the same conditions, that can be enhanced by the addition of promoters or supports [15]. Iron and cobalt-based catalysts, using other metals or oxides as promoters (e.g., ZrO, Ce\({}_{2}\)O\({}_{3}\), Zn), are typically used for the production of hydrocarbons through the RWGS/FTS route, even though precious metals, such as Pt and Pd, are also preferred because they effectively catalyze carbon dioxide activation [20]. The MTH route usually employs copper- and zinc-based catalysts, along with other metals, forming mixed metal oxides (e.g., In, Zr, Al, Ga, Zn) [21]. In both routes, hydrocarbon formation requires the combination of metal oxides with a suitable support in order for the tandem reactions to proceed under 300-400 \({}^{\circ}\)C and 0.5-5.0 MPa [21; 22]. Iron-based materials (e.g., iron oxides and carbides) can be active as catalysts in both RWGS and FTS reactions, promoting olefin formation [15; 23]. The addition of secondary metals, such as zinc (Zn), has been reported to enhance catalytic performance, acting as a structural promoter [24]. Alkali metal ions (e.g., K, Na) can be added to further improve catalytic behavior; potassium (K) addition suppresses methane formation and improves selectivity towards C\({}_{2+}\) products [25], while sodium (Na) addition improves the basicity and the carburization of the surface of the magnetite nanoparticles [26], which are considered critical factors affecting hydrogenation of CO\({}_{2}\) to C\({}_{2+}\) products [27]. Supported iron species can also affect reaction product distribution [28; 29]; supports with high surface area, such as zeolites, carbon, or oxides, provide stabilization of the active sites during the reaction in the presence of water, even at high temperature and/or pressure (i.e., \(>\)200 \({}^{\circ}\)C and \(>\)3 MPa) [30]. Zeolite-like materials or aluminosilicates, such as SAPO-34 or HZSM-5, are used as substrates in CO\({}_{2}\) valorization as they enable surface modification and acidity tuning, as well as catalyze oligomerization, isomerization, and aromatization reactions [13; 31; 32; 33]. The production of hydrocarbons is highly affected by the pore architecture and the acid strength/density, as these catalytic sites enable the conversion of CO\({}_{2}\) activation intermediates that were formed on the metal sites [14]. In fact, the proximity of the iron oxides and the substrate is also important because it is linked with the reaction product selectivity [22]. [PERSON] et al.