(554f) Scalable High Temperature Synthesis of Inorganic Nanoparticles in a Milli-Fluidic Jet Mixing Reactor

The increasing demand for functional inorganic nanoparticles (NPs) necessitates scalable and economically viable manufacturing strategies. The methods impact many areas, including catalysis, energy storage, environmental remediation, and biomedicine. However, translating laboratory-scale syntheses to industrial production poses significant challenges, particularly for inorganic systems requiring precise control over nucleation, growth, and dispersion. Traditional batch synthesis methods, while versatile and widely used in research, often face scale-up limitations because of poor control over mixing, heat transfer, and residence time distribution as reactor volume increases. These constraints typically result in broad particle size distributions (PSD), variations in shape and composition, and reduced performance consistency in downstream applications. Efficient mixing is critical during the rapid nucleation and growth stages of nanoparticle formation, where minor inhomogeneities can lead to significant polydispersity and aggregation.

To overcome these limitations, we have developed a milli-fluidic Jet Mixing Reactor (JMR) designed for high-throughput, scalable synthesis of inorganic nanoparticles under continuous flow. This work explores a novel JMR design featuring an axial flow with two jets impinging on the mainline, resulting in a single stream exiting the reactor. This configuration enables rapid micromixing with characteristic times on the order of milliseconds, significantly faster than typical batch reactors. Our previous work demonstrated the JMR for synthesis of monometallic nanoparticles and complex core@shell nanostructures at room temperature. Of particular interest is the synthesis of Pd@TiO₂ nanoparticles—palladium core particles coated with a titanium dioxide shell—which have shown exceptional catalytic selectivity and activity in biomass upgrading reactions. These nanocatalysts are promising candidates for converting lignocellulosic feedstocks into high-value chemicals and fuels, offering a route to sustainable chemical manufacturing.

In earlier studies, Pd nanoparticles were synthesized separately in a batch process before being introduced into the JMR for TiO₂ shell formation. Whereas effective, this hybrid batch-continuous approach introduces inefficiencies and does not leverage the full potential of continuous flow processing. A major limitation preventing fully continuous synthesis of Pd@TiO₂ nanocatalysts in the JMR has been the requirement of elevated temperatures for Pd particle nucleation.

To address this challenge, we are investigating the performance of the JMR under high-temperature conditions, aiming for a one-step continuous synthesis process. To de-risk the high-cost Pd system and evaluate reactor performance at elevated temperatures, we selected copper (Cu) nanoparticles as a model system. Cu nanoparticles are of considerable interest, particularly in electrocatalytic applications such as CO₂ reduction, where they exhibit the ability to selectively convert CO₂ into hydrocarbons, alcohols, and other renewable fuels.

Initial results from our high-temperature JMR studies using copper as a model system are highly promising. Cu nanoparticles synthesized in the high-temperature JMR using a wet-chemical reduction method exhibited a significantly narrower and more uniform particle size distribution compared to those synthesized via batch processing. In the batch system, Cu nanoparticles were larger than 100 nm in diameter and showed extensive agglomeration. In contrast, Cu nanoparticles synthesized in the high-temperature JMR had an average diameter of approximately 11 nm with much-reduced agglomeration, indicating better control over nucleation and growth dynamics. Characterization of the Cu nanoparticles synthesized in the JMR included transmission electron microscopy (TEM), dynamic light scattering (DLS), and UV-vis spectroscopy. The shape and size change of these nanoparticles also changed when the synthesis temperature increased from 100°C to 150°C; at 100°C, particles are mostly spherical in batch while at 150°C fractals were seen. In JMR, spherical particles were seen at a main line flow rate Q_main- 0.8 mL/min and a jet line flow rate of Q_jet- 0.4 mL/min, whereas nanorods were seen when operated at Q_main- 0.4 mL/min, Q_jet- 0.2 mL/min. Higher flow rates enhance mixing efficiency, resulting in more uniform reactant distribution and higher supersaturation.

Once we achieve Cu NP synthesis, we are extending this high-temperature reactor platform to the Pd@TiO₂ system with the goal of fully integrating Pd nucleation and TiO₂ shell growth in a single, continuous JMR process. This involves using multiple JMRs in series, where the first reactor facilitates Pd nanoparticle formation under high-temperature, reducing conditions, and the second enables TiO₂ shell growth under controlled pH and temperature. Reactor coupling, thermal management, and flow rate balancing are being systematically studied to ensure seamless transition between steps without sacrificing material quality or reactor throughput.

In conclusion, our development of a high-temperature milli-fluidic Jet Mixing Reactor offers a transformative approach to the continuous synthesis of inorganic nanoparticles. Using copper as a model system, we demonstrated superior particle size control, reduced agglomeration, and enhanced scalability compared to conventional batch processes. Future work is focused on copper-based systems for CO₂ conversion, with the goal of enabling sustainable and scalable nanomanufacturing solutions for energy and environmental applications.