(169i) Utilizing Magnetic Heating of Coni Nanoparticles for Electrifying Chemical Conversions

Renewable energy, such as solar for example will be a key player in our transition to a circular economy. However, there are several issues, most notably the intermittent, temporal and geographical nature of the latter. Therefore, efficient and cheap energy storage is essential. Ammonia is a very promising candidate that could facilitate this. It is easier to handle than hydrogen, as it can be stored/transported as a slightly pressurized liquid. Another benefit of ammonia is its volumetric energy density, which also surpasses that of hydrogen (~13 MJ/L). ¹

Ammonia production, at its current technology level, is not compatible with the intermittent nature of solar energy. Due to the high pressures required for dissociative N₂ activation, its production is still centralized in large production plants. Conventional catalytic reactor setups need an additional heat exchanger unit to preheat the gasses before reaching the reactor. However, this step does not allow an on-demand production or decomposition of ammonia. On-demand reactor ignition and operation in stationary conditions require a very rapid heating time, which is made possible by using radiofrequency AC-heating of magnetic nanoparticles. ²

The influence of synthesis conditions on the formation and growth of magnetic nanocomposites was studied using the XRD and HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy). Optimization of the synthesis conditions is followed by a H₂-TPR (hydrogen temperature-programmed reduction) analysis. The latter allows us to optimize the reduction conditions to avoid particle sintering. The reduced material was additionally characterized by ICP-MS and VSM (vibrating sample materials) analysis. The latter sheds some light on the material's magnetic properties and was carried out in different atmospheres and different temperatures. Furthermore, their capacity to heat up in an alternating magnetic field was investigated.

In our work, we prepared magnetic Co_xNi_y nanoparticles/alumina nanocomposites and studied the influence of varying composition (Co:Ni ratio is 2:1, 1:1, 1:2, respectively) on their structural, magnetic and heating properties. CoNi nanoparticles were co-precipitated using NaClO as a precipitant. After hydrothermal treatment, the product was centrifuged and washed. Synthesized nanoparticles were encapsulated within alumina by AlN hydrolysis, followed by reduction at 850 °C for 1 hour in an H₂ flow.

In general, the materials reach higher temperatures at a higher frequency. The heating rate is also better. Our best nanocomposite (Figure 1, right) reached temperatures above 300 °C in less than a minute under magnetic heating while it cooled to ambient temperature in a few minutes when the field was shut down. At 54 mT, a plateau is reached around 60 and 90 °C, at 211 and 293 kHz, respectively. On the other hand, there is a significant difference between the frequencies at 72 mT. At a higher frequency, the material achieved a temperature over 300 °C in 30 s. However, at a lower frequency, it did not reach this temperature even after 175 s. Since the fibre optic temperature sensor is not suitable for higher temperatures (>300 °C), it was not possible to measure the final temperature reached at 91 mT. We were forced to remove the sensor. Nevertheless, the heating curves show a very fast heating rate. 300 °C was reached after 23 and 15 s at 211 and 293 kHz, respectively.

We believe that such rapid heating and cooling under the hydrogen flow offers significantly better flexibility to utilize renewables. We will subsequently decorate the support with metallic Ru nanoparticles and use the material in electrified ammonia production and decomposition.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101022738.

References

(1) Wu, S.; Salmon, N.; Li, M. M.-J.; Bañares-Alcántara, R.; Tsang, S. C. E. Energy Decarbonization via Green H2 or NH3? ACS Energy Lett. 2022, 7 (3), 1021–1033. https://doi.org/10.1021/acsenergylett.1c02816.

(2) Gyergyek, S.; Kocjan, A.; Grilc, M.; Likozar, B.; Hočevar, B.; Makovec, D. A Hierarchical Ru-Bearing Alumina/Magnetic Iron-Oxide Composite for the Magnetically Heated Hydrogenation of Furfural. Green Chem. 2020, 22 (18), 5978–5983. https://doi.org/10.1039/D0GC00966K.

Figure 1: BF and HAADF STEM images of reduced Co₂Ni₁-γ-Al₂O₃ nanocomposite – left. Heating curves of reduced Co₂Ni₁-γ-Al₂O₃ precursor. Measured at different magnetic field strength (black = 54 mT, red = 72 mT, blue = 91 mT) and frequencies (solid line = 211 kHz, dashed line = 293 kHz).