# Synthesis and Characterization of Iron-Based Catalysts for Carbon Dioxide Valorization
## 4 Discussion
K presence seems to cause the same effect on the acid sites, which is more observable for the unsupported samples. Preliminary catalytic tests show relatively low conversion levels for all supported samples (Table 4) under the applied reaction conditions compared to the literature [38],due to the limited catalyst loading allowed by the current experimental setup. Product distribution seems to differentiate over the selected catalysts (Table 4); the Fe\({}_{3}\)O\({}_{4}\)/HZSM-5 reference sample performed better by suppressing CO production while enhancing formation of CH\({}_{4}\), C\({}_{2}\)H\({}_{6}\), C\({}_{3}\)H\({}_{8}\), and C\({}_{3}\)H\({}_{6}\) (26.22% total selectivity towards C\({}_{2+}\) products). The addition of Na further suppresses CO formation, but selectivity to C\({}_{2+}\) products is not improved (17.02%). K presence, which provided stronger basic sites, increased selectivity towards CO and slightly reduced that of CH\({}_{4}\); measured selectivity towards C\({}_{2+}\) products was the lowest (10.16%). The stronger acid sites of the K-Fe\({}_{3}\)O\({}_{4}\) (A)/HZSM-5 do not favor the formation of products with more than two carbon atoms. Over the ZnFe\({}_{2}\)O\({}_{4}\)/HZSM-5 catalyst, despite the fact that CO production is relatively decreased compared to K-Fe\({}_{3}\)O\({}_{4}\) (A)/HZSM-5, selectivity to CO is higher than the Fe\({}_{3}\)O\({}_{4}\) (A)/HZSM-5 and K-Fe\({}_{3}\)O\({}_{4}\) (A)/HZSM-5 samples. Product distribution resembles that of the reference material, but the total selectivity to C\({}_{2+}\) products is 13.78%. Acidity seems to play an important role in C\({}_{2+}\) product formation; unsupported Fe\({}_{3}\)O\({}_{4}\) containing relatively lower acidity and acid sites within the low-medium temperature region (Table S1), compared to the supported samples (Table 3), exhibits an interesting product distribution favoring the formation of C\({}_{2+}\) products, especially that of ethane. Among the supported samples, the addition of Na suppresses CO formation, while that of K generates stronger acid sites that suppress the formation of C\({}_{2+}\) hydrocarbons (Figure S2). Overall, acidity is required to selectively convert CO\({}_{2}\) into higher hydrocarbons; however, our results point out that low-medium acid sites and medium acidity values favor the formation of C-C bonds. This conclusion is in line with recent studies of other research groups [17; 24]. Previous studies [17; 26] have demonstrated that the proximity of Fe\({}_{3}\)O\({}_{4}\)-based catalysts to HZSM-5 matrices influences reaction kinetics, resulting in increased selectivity towards methane (CH\({}_{4}\)) and decreased selectivity towards carbon monoxide (CO) with closer proximity. Consistent with these findings, our catalysts exhibit similar trends (Table 4). As TEM micrographs (Figure 10b,c) show that the iron nanoparticles are well dispersed on the surface of the zeolite micro-particle, the respective morphology is considered of close proximity. Specifically, the catalyst with 100 nm Na-Fe\({}_{3}\)O\({}_{4}\) (A)/HZSM-5 (76% CH\({}_{4}\) and 7% CO), which had closer proximity to the zeolite surface compared to Fe\({}_{3}\)O\({}_{4}\)/HZSM-5 (62% CH\({}_{4}\) and 12% CO), showed enhanced selectivity towards CH\({}_{4}\). Furthermore, we anticipate that the yield to C\({}_{5}\)-C\({}_{11}\) hydrocarbons will increase with appropriate proximity of the catalytic system, although direct measurement of C\({}_{5+}\) hydrocarbons was not feasible due to limitations in our in situ gas measurement setup. Higher hydrocarbons would be analyzed ex situ by GC-MS spectroscopy; for current sampling, a collection/condensation bottle was in line to the reactor exit, exhibiting fog presence, which was not sufficient for collection and analysis. Fog presence suggests that additional unidentified products are formed, in line with the calculated carbon balance (97 \(\pm\) 2%).