Kinetics study to identify reaction-controlled conditions for supercritical hydrothermal nanoparticle synthesis with flow-type reactors

• Rapid mixing and high temperature decrease the CeO2 nanoparticle size.
• Ranges of Reynolds number for reaction-controlled condition depend on temperature.
• The rate constant including the effect of drop in dielectric constant was estimated.
• The threshold of Damköhler number for reaction-controlled conditions was identified.

Flow-type reactors are effective for the precise control of reaction conditions and high throughput production. To enhance the effectiveness of this process, the establishment of a design method is required. For this purpose, the effects of operating parameters on supercritical hydrothermal nanoparticle synthesis in a flow-type reactor were examined. Ceria nanoparticles were formed from 2.0 mM cerium nitrate at reaction temperatures ranging from 200 to 380 °C and with a flow rate of 11.6–37.5 mL/min. In addition, channel sizes of 0.3, 1.3, and 2.3 mm were used for the mixing point. Rapid mixing and higher temperatures were found to enable the formation of smaller nanoparticles. Furthermore, all experimental results were summarized using dimensionless numbers. Though the Reynolds number was related to the effect of mixing on particle formation, this number is independent of the reaction rate. Results were correlated using the Damköhler number, the ratio of reaction rate to mixing rate. From the threshold value of the Damköhler number, reaction-controlled conditions where the particle size was independent of the flow/mixing rate could be predicted.

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