Imaging the inside of a continuous nanoceramic synthesizer under supercritical water conditions using high-energy synchrotron x-radiation

Continuous hydrothermal flow synthesis (CHFS) offers a controllable route to the production of nanocrystalline materials. We describe the application of tomographic X-ray methods to image, for the first time, crystallization at the interior of an in operando CHFS reactor. In the experiment, the steady-state formation of nanoparticulate CeO2 was followed: the synthesis proceeds by rapid hydrolysis of cerium ammonium nitrate and hydrothermal coprecipitation in a near/super-critical water environment (T = 340−450 °C, P = 24 MPa). The results identify the location of particle growth with accompanying indications of crystallite size, and also reveal the build-up of material on the reactor wall during long syntheses. The imaging represents a significant achievement in that information of this kind can be gleaned from such an inhospitable environment as that of a CHFS reactor. The novel combination of tomographic angle- and energy-dispersive diffraction employed was particularly appropriate for this in situ study, where the CHFS apparatus could not be rotated (as is required in conventional tomography methods). This imaging capability offers new insights into the synthesis process, which can lead to optimization, better reactor design, and adaptation toward industrial application.