Carbon-Supported Magnetic Iron Oxide Composites: Vortex Electromagnetic Field-Assisted Microstructural Engineering, Physicochemical Characterization, and CO2 Hydrogenation Performance

The development of efficient and recoverable catalysts is essential for advancing carbon dioxide utilization technologies and improving the sustainability of catalytic processes. Magnetic iron oxide-based composites supported on carbon materials are particularly promising due to the combination of catalytic activity, structural stability, and the possibility of magnetic separation from the reaction medium. The aim of this study was to produce potassium-modified magnetic iron oxide composite catalysts based on Fe3O4 and CoFe2O4 on carbon supports and to study the influence of synthesis conditions and composition on their phase composition and structural characteristics. Based on a comparative analysis of the obtained data, the catalyst composition was optimized and a preliminary assessment of their application in CO2 hydrogenation was conducted. Potassium-modified magnetic iron-containing composites supported on carbon nanotubes (CNTs) and coconut shell activated carbon (AC) were synthesized via coprecipitation and vortex electromagnetic field (VEF) synthesis followed by thermal treatment and evaluated as promising materials for CO2 utilization processes. The influence of synthesis parameters, including pH, Fe2+/Fe3+ molar ratio, coprecipitation temperature, and calcination temperature, on the structural, phase, and magnetic properties of the catalysts was studied. Structural analysis confirmed the formation of Fe3O4 and CoFe2O4 spinel phases. The optimal calcination temperature of 673 K ensured the formation of a well-crystallized Fe3O4 phase with sufficiently high saturation magnetization, whereas a further increase in temperature resulted in partial oxidation of Fe3O4 with the formation of Fe2O3. Optimal synthesis conditions were determined ensuring uniform deposition of Fe3O4 nanoparticles measuring 10–100 nm on carbon supports. It was found that CoFe2O4-based composites possess high coercivity but are characterized by lower catalytic activity in the CO2 hydrogenation reaction: the maximum CO2 conversion for CoFe2O4–K/CNTs was 59% at 600 °C, which is lower than that of the studied iron oxide composites. Synthesis using VEH (vortex electromagnetic homogenizer) improved particle dispersion and catalytic performance, increasing apparent CO2 conversion from 51% to 54% at 450 °C. Methane formation was more pronounced at intermediate temperatures, whereas at temperatures above 500 °C the product distribution shifted toward CO, indicating the increasing contribution of the reverse water–gas shift (RWGS) reaction. It was shown that optimization of the synthesis parameters made it possible to obtain magnetic composites with controlled structural-phase and magnetic properties, and the combination of high dispersion, structural stability and magnetic activity makes the synthesized composites promising catalysts for carbon dioxide hydrogenation processes.