Scaling-up a Confined Jet Reactor for the Continuous Hydrothermal Manufacture of Nanomaterials

A confined jet reactor (mixer) is presented as a novel solution for the scalable continuous hydrothermal flow synthesis (CHFS) of nanoceramics. In CHFS, nanoceramics are formed upon mixing of two streams consisting of an aqueous metal salt solution at room temperature with a flow of less dense supercritical water (at 240 bar and 450 °C). Upon mixing, hydrolysis and dehydration occurs, resulting in the particles being formed in a continuous manner. The confined jet mixer used herein overcomes previous designs of mixers that can accumulate material internally and block. A method for scaling up the jet mixer (reactor) is described, to determine the size of jet mixer (internal mixer diameter 13.5 mm) prior to its use in a newly commissioned pilot plant designed to process flow rates 40 times greater than the equivalent laboratory-scale process (internal mixer diameter 4.6 mm). It was confirmed that the pilot plant scale mixer allowed safe and continuous operation with no blockages at much higher concentrations (i.e., higher molarity) of metal salt precursor than laboratory scale because of the higher velocities and larger physical dimensions of the mixer. Consequently, the pilot plant was used to manufacture nanoparticles at a rate >400 times that of the laboratory-scale process. The synthesis of zinc oxide nanoparticles was used as a model to compare the properties of particles produced on different production scales. The same model system was also used to assess the limitations of a scale-up strategy based on mass (i.e., increasing the molarity of the metal salt)

Scale Up Production of Nanoparticles: Continuous Supercritical Water Synthesis of Ce–Zn Oxides

A new continuous supercritical water pilot plant was used for the large-scale production of nanomaterials in the Zn–Ce oxide system. Similar to an existing laboratory continuous process, the pilot plant mixes aqueous solutions of the metal salts at room temperature with a flow of supercritical water (450 °C and 24.1 MPa) in a confined jet mixer, resulting in the formation of nanoparticles in a continuous manner. The Zn–Ce oxide system, as synthesized here under identical concentration conditions than those used in our laboratory scale process (but 17.5 times total flow rate), has been used as a model system to identify differences in particle properties due to the physical enlargement of the mixer. The data collected for the nanoparticles from the pilot plant was compared to previous work using a laboratory scale continuous reactor. In the Ce–Zn binary oxide series, it was shown that Zn had an apparent solubility of about 20 mol% in the CeO₂ (fluorite) lattice, whereafter a composite of the two phases was obtained, consistent with the high solubility observed in previous studies using a continuous hydrothermal process. Because of the inherent scalability of the continuous process and excellent mixing characteristics of the confined jet mixer, it was found that the pilot plant nanoparticles were almost indistinguishable from those made on the laboratory scale

A Direct and Continuous Supercritical Water Process for the Synthesis of Surface-Functionalized Nanoparticles

A new processing methodology is presented for the direct synthesis of surface-functionalized nanoparticles through modification of a single-step continuous supercritical water process. The processing methodology utilizes inexpensive metal salt precursors that form nanoparticles upon mixing the metal salt solution with a supercritical water flow (24 MPa and 450 °C). Surface functionalization is achieved through introducing a supplementary flow of capping agent (citric acid in this example) to the stream of nascent (newly formed) nanoparticles using a novel reactor design. It was found that certain process attributes were key to effective functionalization of the nascent nanoparticle stream, and that high grafting densities of the capping agent were obtained in a relatively narrow process window. We have also used the core design of the reactor to devise and test a scale-up methodology to produce large quantities of surface-functionalized nanoparticles. A method for scaling-up the reactor is described, using a newly developed pilot plant designed to process flow rates 20× greater than the equivalent laboratory-scale process, which yields products at rates of ca. 1 kg/h (effectively semi-industrial-scale production). The method enables large-scale production without recourse to expensive or environmentally damaging reagents and uses water as the only process solvent, a significant advantage over many methods commonly used to produce surface-functionalized nanoparticles. We report the synthesis and characterization of citrate-functionalized Fe₃O₄ nanoparticles as a model system and present detailed characterization of the materials obtained at both processing scales

Structure–Property–Composition Relationships in Doped Zinc Oxides: Enhanced Photocatalytic Activity with Rare Earth Dopants

In this paper, we demonstrate the use of continuous hydrothermal flow synthesis (CHFS) technology to rapidly produce a library of 56 crystalline (doped) zinc oxide nanopowders and two undoped samples, each with different particle properties. Each sample was produced in series from the mixing of an aqueous stream of basic zinc nitrate (and dopant ion or modifier) solution with a flow of superheated water (at 450 °C and 24.1 MPa), whereupon a crystalline nanoparticle slurry was rapidly formed. Each composition was collected in series, cleaned, freeze-dried, and then characterized using analytical methods, including powder X-ray diffraction, transmission electron microscopy, Brunauer–Emmett–Teller surface area measurement, X-ray photoelectron spectroscopy, and UV–vis spectrophotometry. Photocatalytic activity of the samples toward the decolorization of methylene blue dye was assessed, and the results revealed that transition metal dopants tended to reduce the photoactivity while rare earth ions, in general, increased the photocatalytic activity. In general, low dopant concentrations were more beneficial to having greater photodecolorization in all cases

Gas Sensing with Nano-Indium Oxides (In₂O₃) Prepared via Continuous Hydrothermal Flow Synthesis

A rapid, clean, and continuous hydrothermal route to the synthesis of ca. 14 nm indium oxide (In₂O₃) nanoparticles using a superheated water flow at 400 °C and 24.1 MPa as a crystallizing medium and reagent is described. Powder X-ray diffraction (XRD) of the particles revealed that they were highly crystalline despite their very short time under hydrothermal flow conditions. Gas sensing substrates were prepared from an In₂O₃ suspension via drop-coating, and their gas sensing properties were tested for response to butane, ethanol, CO, ammonia, and NO₂ gases. The sensors showed excellent selectivity toward ethanol, giving a response of 18–20 ppm

High-Throughput Continuous Hydrothermal Synthesis of Nanomaterials (Part II): Unveiling the As-Prepared Ce_<i>x</i>Zr_<i>y</i>Y_<i>z</i>O_2−δ Phase Diagram

High-throughput continuous hydrothermal flow synthesis was used to manufacture 66 unique nanostructured oxide samples in the Ce–Zr–Y–O system. This synthesis approach resulted in a significant increase in throughput compared to that of conventional batch or continuous hydrothermal synthesis methods. The as-prepared library samples were placed into a wellplate for both automated high-throughput powder X-ray diffraction and Raman spectroscopy data collection, which allowed comprehensive structural characterization and phase mapping. The data suggested that a continuous cubic-like phase field connects all three Ce–Zr–O, Ce–Y–O, and Y–Zr–O binary systems together with a smooth and steady transition between the structures of neighboring compositions. The continuous hydrothermal process led to as-prepared crystallite sizes in the range of 2–7 nm (as determined by using the Scherrer equation)

Combined EXAFS, XRD, DRIFTS, and DFT Study of Nano Copper-Based Catalysts for CO₂ Hydrogenation

Highly monodispersed CuO nanoparticles (NPs) were synthesized via continuous hydrothermal flow synthesis (CHFS) and then tested as catalysts for CO₂ hydrogenation. The catalytic behavior of unsupported 11 nm sized nanoparticles from the same batch was characterized by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and catalytic testing, under CO₂/H₂ in the temperature range 25–500 °C in consistent experimental conditions. This was done to highlight the relationship among structural evolution, surface products, and reaction yields; the experimental results were compared with modeling predictions based on density functional theory (DFT) simulations of the CuO system. In situ DRIFTS revealed the formation of surface formate species at temperatures as low as 70 °C. DFT calculations of CO₂ hydrogenation on the CuO surface suggested that hydrogenation reduced the CuO surface to Cu₂O, which facilitated the formation of formate. In situ EXAFS supported a strong correlation between the Cu₂O phase fraction and the formate peak intensity, with the maxima corresponding to where Cu₂O was the only detectable phase at 170 °C, before the onset of reduction to Cu at 190 °C. The concurrent phase and crystallite size evolution were monitored by in situ XRD, which suggested that the CuO NPs were stable in size before the onset of reduction, with smaller Cu₂O crystallites being observed from 130 °C. Further reduction to Cu from 190 °C was followed by a rapid decrease of surface formate and the detection of adsorbed CO from 250 °C; these results are in agreement with heterogeneous catalytic tests where surface CO was observed over the same temperature range. Furthermore, CH₄ was detected in correspondence with the decomposition of formate and formation of the Cu phase, with a maximum conversion rate of 2.8% measured at 470 °C (on completely reduced copper), supporting the indication of independent reaction pathways for the conversion of CO₂ to CH₄ and CO that was suggested by catalytic tests. The resulting Cu NPs had a final crystallite size of ca. 44 nm at 500 °C and retained a significantly active surface