(560hp) Coupling the Magnetic and Catalytic Properties of Fe3O4 Via Shape-Controlled Routes and Cr Doping

Iron catalysts play a vital role in the leading chemicals industry, favoring the production of ammonia, hydrogen, and liquid hydrocarbons through the Haber-Bosch, Water-Gas Shift, and Fischer-Tropsch processes, respectively. These catalysts are benefited by an increase in surface area, which can be achieved through reduced dimensionality. To produce efficient catalysts, thermal decomposition routes capable of controlling size and shape of nanoparticles have been reported, enabling fine tuning of the catalytic properties of a given material. Consequently, for iron oxide (Fe₃O₄) nanoparticles, for example, structural control also enables improved magnetic properties. In the context of catalysis, Fe₃O₄ is a promising material due to its ability to generate localized heat by the conversion of electromagnetic radiation into heat, as seen in magnetic hyperthermia. Therefore, combining the properties of iron catalysts with synthetic control and doping have the potential to make an impact in the chemicals industry.

In this work, thermal decomposition is used to produce monodisperse, spherical, cubic and octahedral shaped, Fe_(3-x)Cr_xO₄ nanoparticles as a strategy to improve the activity of the iron oxide (Fe₃O₄) host nanoparticles via Cr⁺³ doping, while maintaining the magnetic properties of the host lattice. Cr⁺³ replaces Fe⁺³ in the octahedral sites of Fe₃O₄ inverse spinel structure at low doping concentrations (x < 15 mol%), and tetrahedral and octahedral sites are substituted at higher concentrations. Low doping concentration improves activity and redox performance due to coupling of the Fe⁺³/Fe⁺² with Cr⁺²/Cr⁺³, and regeneration of Fe⁺² on the surface. To confirm this regeneration effect and the site occupancy of Cr in Fe₃O₄, L-edge X-ray Absorption Spectra of Fe and Cr is collected as well as X-ray Photoelectron Spectroscopy (XPS), to determine the formation of oxygen vacancies and to probe the valence state of Cr and Fe. In addition, Cr doping changes the pore structure from mesopores to micropores, thereby increasing the surface area, demonstrated through BET measurements. Finally, magnetic properties are characterized via Superconducting Quantum Interference Device (SQUID) measurements to show how the saturation magnetization, and coercivity are preserved. Induction heating is characterized by determining the Specific Loss Power of these materials under various magnetic field strength.