Characterization of Magnetoelectrically Driven Catalytic Reactions by COMSOL Multiphysics® Parametric Studies
In heterogeneous catalysis, magnetic nanostructures have been mainly used as magnetically recoverable catalysts. Recently, we have shown that magnetic fields can be utilized to initiate polarization-driven catalytic reactions on surfaces of multiferroic nanoparticles (NP) via direct magnetoelectric (ME) effect.
For instance, these ME nanocatalysts were able to successfully decompose relevant organic contaminants upon application of magnetic fields. In experiments, we have found that the catalytic effect can be tuned as a function of the magnetic field strength and frequency. With this work, we aim to model this complex catalytic behavior to gain a deeper understanding of the catalytic system and to improve future nanocatalyst designs. The magnetoelectric nanocatalyst consists of magnetostrictive CoFe2O4 (CFO) core coated with multiferroic BiFeO3 (BFO) shell. Said NPs are immersed in a water solution of a model organic dye such as Rhodamine B (RhB). Upon application of alternating magnetic field, the organic contaminant is degraded over time. The physics of our COMSOL Multiphysics® model used to describe the ME effect was defined using the Magnetic Fields (mf), Solid Mechanics (solid), Electrostatics (es), Chemistry (chem), and Transport of Diluted Species (tds) interfaces. For the simulation, octahedral CFO NP with a diameter of 30 nm and an epitaxially grown BFO shell with a thickness of 5 nm were considered in the model. The magnetic field was applied to the boundaries of the medium surrounding the nanocatalyst. COMSOL Multiphysics® was used to compute the magnetization gradients within the material by using the applied magnetic field's corresponding magnetization values from the measured VSM hysteresis curves. The internal strain generated in a CFO NP under magnetic fields was governed by the following equation λz=1.5⋅λs(Mz/Msat), where λz is the strain along the z-axis, λs the magnetostriction parameter, Mz the magnetization along the z-axis and Msat the saturation magnetization of the CFO core. The strain transfer from CFO core to the BFO shell was assumed to be ideal. This strain in the BFO shell is converted into electric polarization on the surface of BFO. Furthermore, by addition of a simplified model for the time-dependent chemical surface reactions with transport kinetics in the vicinity of the surface of the CFO-BFO NP we will be able to demonstrate the degradation kinetics. This calculation assumes that the kinetics of RhB degradation reaction catalyzed by the CFO-BFO NP are (pseudo)-first-order reactions. We have found that the catalytic effect of ME NPs is due to piezoelectrically induced water splitting and subsequent formation of hydroxyl radicals. The transient surface charges on the BFO shell can be directly mapped to the production of reactive radical species which react with the micropollutant species immediately, due to the short lifetime, to form degraded inert carbon species in solution. The proposed method will still illustrate the mechanism in detail and this way reveal new insights into the characteristics of magnetoelectrically driven catalytic reactions with this nanocatalyst.
