(80c) Surface Characterization of Modified Fe3O4 Catalysts for Inductively Driven Alcohol Oxidation

Magnetic nanoparticles (NPs) such as iron oxide (Fe₃O₄) generate local heating under an alternating magnetic field (AMF), and they have been used for magnetic hyperthermia, drug delivery, and catalysis. Induction heating (IH) catalysis has shown to be a prominent field due to the potential decrease in heat dissipation losses. Since the catalyst is also the heat source, energy transfer to reactants is facilitated. Optimum Fe₃O₄ nanocatalysts are generally synthesized with the aid of surfactants, which allows tuning of catalyst morphology and size (surface area). However, the application of such magnetic catalysts presents a few challenges, namely 1) accurate determination of the surface temperature (i.e., reaction temperature) and 2) efficient surfactant removal that hinders reactant accessibility and heat transfer, 3) catalytic deactivation due to coking/oxidation of Fe₃O₄. Despite advances in the development of thermal probes that are not susceptible to an AMF, it is still challenging to quantify the NP's surface temperature instead of relying on bulk measurements to quantify heat transfer, which is key in catalysis. This work uses a photoluminescent (PL) shell to determine the catalyst's surface temperature in situ using europium doped yttrium vanadium oxide (YVO₄:Eu³⁺). By incorporating this shell on the NPs, the surface temperature can be quantified based on the change in PL intensity with temperature. Afterward, the Fe₃O₄ surface temperature can be calibrated to a set AMF to study the role of IH on the condensation of 1-octanol. The catalyst is then modified with respect to its surface chemistry, morphology, and doping concentration (Fe_3-xCr_xO₄) to evaluate the effects on reaction selectivity and yield.

This work demonstrates that the reaction of 1-octanol on Fe₃O₄ structures can be tuned to increase aldehyde yield by 2.5-fold via modification of the surface ligands and by performing the reaction inductively. Notably, product selectivity also changes to favor the production of the octanol ester once highly faceted and asymmetric star-shaped nanoparticles are used. Additionally, the catalyst's redox properties can be tuned via Cr doping, modifying the IH profile and yields/activities. These studies are facilitated by the determination of the surface temperature, where the sample is placed in an AMF combined with a PL spectrometer using fiber-optic light guides. The change in PL intensity with an applied field is compared to a similar analysis using a controlled temperature stage. The surface temperatures are up to 65 °C higher than bulk, requiring adjustment of the AMF for IH catalysis at the desired reaction temperature (200 °C). The core NPs are surface treated to allow accessibility of the reactants, and interestingly, this treatment also enhances heat transfer. As a comparison, surfactant-less NPs are also tested but are outperformed by phase-transferred colloidal NPs. The product yield with treated colloidal NPs increases by 6-fold (IH) and 2.5-fold (thermal heating) compared to surfactant-less. These results demonstrate the advantages of using IH technology for efficient energy consumption. Furthermore, the incorporation of Cr has the potential to stabilize Fe⁺² (Fenton active species), expanding the applications of this catalyst to other reactions and potentially decreasing the rate of Fe²⁺ oxidation to weakly magnetic and lower IH temperature catalysts, which improves its lifetime.