(70e) Functionalizing Fe3O4 Nanoparticles for Local Luminescence Probing and Mediation of Heat Transfer in Induction Heating Catalysis

Nanoparticle surface chemical transformations have emerged as technological solutions to catalysis, bio-sensing, drug-delivery, and optical enhancement. Among these applications, the utilization of magnetic nanoparticles, such as iron oxide (Fe₃O₄), provide an extra degree of freedom to particle functionality. For example, magnetically recoverable catalysts facilitate catalyst regeneration and recycling, in addition to providing local heating via Radio Frequency (RF) conversion to heat at the nanoparticle surface. However, optimum heat generation is dictated by the particle size and shape, requiring surface passivation with surfactants such as oleic acid (OA) during the synthesis. Unfortunately, the utilization of such ligands hinders induction heating catalysis applications by limiting the available surface area (SA) and acting as an insulating layer to heat transfer. Consequently, the catalysts require larger alternating magnetic fields (AMF) to generate the required temperatures in the reaction media, while adsorbates are overheated, leading to coking and deactivation. Current solutions for nanoparticle surface treatment entails phase transfer procedures, and ligand replacement, in which neither results in SA enhancement. In contrast, the development of thermal probes that are not susceptible to an AMF, still require concentrated solutions of nanoparticles or provide indirect surface temperature measurements with thermo-sensitive molecules that are unable to withstand high temperatures (>70 °C) without degradation. This work aims to address the challenges in developing tunable induction heating catalysts by surface functionalization, allowing the determination of the surface temperature of these nanoparticles in situ, and increasing the available SA with ligand displacement.

In this work, morphologically-controlled Fe₃O₄ particles, prepared via colloidal decomposition, were treated with pyridine and tetramethylammonium hydroxide (TMAOH) to determine the extent of surfactant removal. The effect of ligand binding on different Fe₃O₄ morphologies was investigated with Inelastic Neutron Scattering (INS) to facilitate the distinction between oleate ions and OA, and avoid the interference of IR absorption by the nanoparticle on signal analysis and surfactant detection. Through the understanding of the OA binding, improved heat generation and transfer to adsorbates were possible, resulting in a 4x increase in surface area and a 2x increase in heat generation, measured in terms of Specific Loss Power (SLP). Simultaneously, a europium doped sodium yttrium fluoride (NaYF₄: Eu³⁺) shell was deposited on Fe₃O₄ nanoparticles of similar size and shape, to determine the surface temperature based on the change in luminescent intensity/lifetime under an applied AMF. The luminescent probe allows high-resolution thermal monitoring without the need for concentrated nanoparticle solutions or macroscopic temperature increase. This method is used to standardize surface temperature to the nanoparticle structural properties and applied field, ultimately providing control of the selectivity of induction heating catalysis. Consequently, dehydrogenation of alcohols on Fe₃O₄ resulted in aldehyde selectivity and a twofold yield on cubic nanostructures compared to spheres, while non-morphologically controlled particles resulted in ester formation.