Supercritical Hydrothermal Synthesis Research Articles

Innovative design and experimental research on the world's first pilot plant for continuous supercritical hydrothermal synthesis nano copper

Supercritical hydrothermal synthesis (SCHS) is a promising nanomaterial preparation technology. We have successfully constructed the world's first continuous supercritical hydrothermal synthesis of nano copper pilot plant, with a yield of 100 kg/d and a grain size of nano copper products between 48–90 nm. In this work, the progressiveness of the supercritical hydrothermal synthesis pilot plant is systematically introduced, including rapid and uniform mixing, high Reynolds number, precise control of reaction time, avoidance of blockage and corrosion, and automation and safety control of the pilot plant. In addition, the synthesis mechanism and experimental results of supercritical hydrothermal synthesis of nano copper are further introduced. Finally, we analyze the economy of the pilot plant. The experimental research on the continuous hydrothermal synthesis of nano copper on a pilot scale may provide some guidance for future commercialization.

Supercritical hydrothermal synthesis of ultra-fine Cu powders

ABSTRACT Ultra-fine Cu powders were synthesized using supercritical water as a reaction medium and formic acid as a reducing agent. The effects of reducing agent, reaction temperature, and complexing agent on the phase constituents, particle size distributions and morphology of the synthesized ultra-fine Cu powders were investigated. A complex mixture of Cu2O and Cu powders formed by pure supercritical water while high purity homogeneous ultra-fine Cu powders can be synthesized by supercritical water with formic acid as a reducing agent. Moreover, fine tuning of particle size and morphology can be realized by the addition of Ethylenediamine tetraacetic acid (EDTA). The modification mechanism of EDTA on the surfaces of ultra-fine Cu powders was investigated by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. EDTA adsorbed on the surfaces of nanoparticles by the dehydration reaction of carboxyl groups in the molecule of EDTA and hydroxyl groups on surfaces of copper nanoparticles (CuOH).

Response surface study on continuous supercritical hydrothermal synthesis of nano-zirconia: Scale-up from laboratory to industrialization

Commercial interest in nanomaterials and their applications lead to the development of many new methods for mass production. Among them, supercritical hydrothermal synthesis (SCHS) is a highly efficient and environmental-friendly technology. In this work, an advanced industrialized device for SCHS was used to verify the synthesis of nano-zirconia particles. The influencing laws and mechanisms of different material parameters (precursor type, precursor concentration, and pH) and process parameters (reaction temperature, reaction pressure, residence time, and Reynolds number) of nano-zirconia synthesis were studied. For the first time, the state changes of zirconium precursors and the precipitation behavior of nano-zirconia particles were in-situ observed by a high-speed camera. Central composite design (CCD) of response surface methodology (RSM) was adopted to investigate interactive effects of reaction temperature (200–420 °C), reactant concentration (0.01–0.6 mol L−1), alkali ratio (0–5) and flow rate (3–15 mL min−1), with particle size (APS), crystal form ratio (k) and reactant conversion rate (α) serving as response values. The influence of key process parameters follows an order of reaction concentration > reaction temperature > system flow > alkali ratio for APS, alkali ratio > system flow > reaction temperature > reaction concentration for k, and reaction temperature > alkali ratio > system flow > reaction concentration for α, respectively. The optimal process parameters were then obtained and further verified at lab and industrial scale. Zirconia powders with particle sizes of 4.98 nm and 8.74 nm were synthesized, respectively, indicating five hundred times magnification of continuous SCHS of nano zirconia from laboratory to industrialization.

Study on the Synthesis of Nano Zinc Oxide Particles under Supercritical Hydrothermal Conditions.

The supercritical hydrothermal synthesis of nanomaterials has gained significant attention due to its straightforward operation and the excellent performance of the resulting products. In this study, the supercritical hydrothermal method was used with Zn(CH3COO)2·2H2O as the precursor and deionized water and ethanol as the solvent. Nano-ZnO was synthesized under different reaction temperatures (300~500 °C), reaction times (5~15 min), reaction pressures (22~30 MPa), precursor concentrations (0.1~0.5 mol/L), and ratios of precursor to organic solvent (C2H5OH) (2:1~1:4). The effects of synthesis conditions on the morphology and size of ZnO were studied. It was found that properly increasing hydrothermal temperature and pressure and extending the hydrothermal time are conducive to the more regular morphology and smaller size of ZnO particles, which is mainly achieved through the change of reaction conditions affecting the hydrothermal reaction rate. Moreover, the addition of ethanol makes the morphology of nano-zno more regular and significantly inhibits the agglomeration phenomenon. In addition to the change in physical properties of the solvent, this may also be related to the chemical bond established between ethanol and ZnO. The results show that the optimum synthesis conditions of ZnO are 450 °C, 26 MPa, 0.3 mol/L, 10 min, and the molar ratio of precursor to ethanol is 1:3.

Size-controlled synthesis of copper oxide nanoparticles with a supercritical hydrothermal synthesis method

Supercritical hydrothermal synthesis (SCHS) is a promising technique for the preparation of copper oxide nanoparticles with fast reaction rate, green efficiency and low cost. In this paper, nanoparticles of copper oxide ranging from 53.0 to 94.0 nm were synthesized by supercritical hydrothermal method. The effects of different precursor concentrations, alkali addition, reaction pressure, reaction temperature and reaction time on the particle size, morphology and particle uniformity of copper oxide nanoparticles were also investigated, and the mechanism of each parameter was analyzed. The growth kinetic equation of copper oxide nanoparticles under supercritical hydrothermal conditions was established by analyzing the influence laws of temperature and reaction time on the grain size of the products. Further, the growth activation energy and the growth mechanism controlled by the grain surface process was obtained. The formation mechanism of copper oxide nanoparticles in rod form was analyzed by TEM results.

Continuous supercritical hydrothermal synthesis of stabilized ZrO2 nanocomposites: Doping mechanism of typical metals and transition elements

In this study, Mg, Ce, Y and La-doped nano-zirconia particles were successfully obtained by continuous supercritical hydrothermal synthesis combined with nanotechnology and crystal stabilization technology. Through detailed characterization and analysis of the structure, crystal form, grain size of the particles and by-products, the doping amount of different element dopants was optimized, and the stable pathway of tetragonal and cubic phases was proposed. Moreover, the doping mechanisms of different elements and phase diagram of doped nano-zirconia were revealed. The results show that the crystal form is determined by the doping amount, ionic radius and intrinsic properties of ions. Y and Ce doping can completely stabilize the tetragonal or cubic phase without the formation of by-products. Mg and La doping produce by-products of Mg(OH)2 and La2O2CO3. As the ionic radius increases, the radius mismatch with Zr4+ leads to a stronger ability to stabilize the tetragonal or cubic phase. At the same time, minimum dopant for stable tetragonal or cubic phases will decrease.

Supercritical hydrothermal synthesis of copper nanoparticles: Experimental and kinetic study

Copper nanoparticles are widely used in electronics, catalysis, energy storage and other fields. But the traditional preparation technology has the defects of high cost, low yield, and poor product dispersion, which prevent industrialization and application. Supercritical hydrothermal synthesis (SCHS) technology is a green frontier technology for the preparation of nanomaterials, which can overcome the deficiencies of the existing technology and achieve industrialization. In this paper, the material system of SCHS of copper nanoparticles is optimized, and the effects of precursor, reductant, reductant dosage and PVP dosage on the particle size and morphology of the products are investigated, and the influence mechanism is analyzed. In addition, the effect of PVP dosage on the antioxidant capacity of copper nanoparticles was investigated in the context of the oxidation kinetics of copper nanoparticles. The optimal material system for the synthesis of antioxidant copper nanoparticles was finally obtained.

Cooling rate of reactant solution in a flow-type supercritical hydrothermal reactor estimated using neutron radiography

Rapid cooling of reactants quenches the intermediate state of chemical reactions and enables the synthesis of novel materials that cannot be realized by thermodynamic equilibrium processes. In this study, we estimated the cooling rate of reactant solutions during the supercritical hydrothermal synthesis of metal oxide nanoparticles. Neutron radiography measurements were performed to visualize the profile of the average water density at a junction where a stream of heated water was mixed with another stream of cooling water. The cooling rate of the heated water that mimicked the reactant solution was estimated using the relationship between the density and temperature of water under constant pressure conditions. This result should provide vital information about the flow-type synthesis of novel metal oxide nanoparticles.

Supercritical hydrothermal synthesis of nano-ZnO: Effects of key parameters and reaction mechanism

In this paper, ZnO particles with the particle size as low as 23 nm were successfully prepared by supercritical hydrothermal synthesis technique. The particle size of nano-ZnO decreased with the increase of temperature and pressure. Nano-ZnO particles in ZnSO4 system were spherical with smaller particle size. The discrepancy in nano-ZnO produced by different precursor of Zn(NO3)2, Zn(CH3COO)2 or ZnSO4 is attributed to the anion effects and supersaturation. The particles in the KOH system (29 nm) were smaller than those in the NaOH system (44 nm). For precursor concentration, intermediate Zn(OH)2 was generated at lower concentration, while Zn4SO4(OH)6·4H2O was produced at higher concentration. For alkali concentration, as the gradual increase of KOH concentration, Zn(OH)2 began to decrease and gradually transformed into Zn(OH)3– and [Zn(OH)4]2−. When the KOH concentration reached a certain value, [Zn(OH)4]2− occupied the dominance in the mixed solution.

Nanocrystalline HZSM‐5prepared by supercritical hydrothermal synthesis and its catalytic esterification performances

AbstractThe nanocrystalline HZSM‐5 was synthesized by supercritical hydrothermal synthesis for the first time and investigated in catalytic esterification of pyromellitic dianhydride(PMD)and n‐butanol to tetrabutyl pyromellitate(TBPM). The HZSM‐5 prepared by conventional hydrothermal synthesis was also examined for comparison. The catalysts were studied using XRD, N2 adsorption, NH3‐TPD, Pyridine‐adsorption FT‐IR, SEM,[27] Al MAS NMRandCHNS analysis. According to the results of characterization, the supercritical hydrothermal condition was helpful to reduce the size of crystal particles and improve the crystallinity of HZSM‐5, and shorten the crystallization time. The surface area, especially external surface area, and pore volume were increased, and the total acidity was increased. The strong acid sites were transformed into the weak acid sites, and the strength of weak and strong acid was weakened. The amount of framework tetrahedral Al species and Brönsted acid sites were increased. Therefore, the formation of coke was retarded, and thePMD conversion, the TBPM selectivity and the catalytic stability were obviously improved.

Preparation of CuO nanorods for lithium-ion battery anodes by supercritical hydrothermal reaction of spent acidic etchant

An economical and environmentally friendly treatment method is urgently needed for the copper-containing spent acidic etchant produced by printed circuit boards. In this work, CuO nanorods with ~30 nm in diameter and ~100 nm in length were prepared from spent acidic etchant by supercritical hydrothermal synthesis. The structures and morphologies of spent acidic etchant-derived CuO (E-CuO) and analytical pure CuCl2-derived CuO (P-CuO) were characterized. The E-CuO has more impurities such as Fe and Zn than the P-CuO. The energy storage performance of the prepared CuO anodes was tested. The E-CuO delivers a high initial discharge capacity of 793.3 mAh·g−1 and a better rate and cycling performance than the P-CuO due to the smaller particles and the doping of Fe and Zn from the spent acidic etchant in the E-CuO.

Supercritical Hydrothermal Reactions for Material Synthesis

Abstract Since the early 1990s, Adschiri and his colleagues have performed research on the use of supercritical water in diverse applications, including heavy oil reforming, waste polymer decomposition and chemical raw materials recovery, cellulose hydrolysis and sugar recovery, lignin decomposition and chemical raw materials recovery, and nanoparticle synthesis. Regarding inorganic materials synthesis, they invented supercritical hydrothermal synthesis for the continuous flow synthesis of metal oxide nanoparticles, which has already found worldwide industrial applications. They proposed a two-fluid mixing flow system where an aqueous metal salt solution is mixed with supercritical water to heat the solution to the supercritical state within a very short time, that is not attainable by conventional reactor systems. Organic-inorganic hybrid nanoparticles synthesis was demonstrated in the supercritical state, realizing homogeneous phase formation of organic molecules at high loadings. By optimizing the reaction conditions, small facet-controlled nanoparticles with narrow particle size distribution can be obtained. Organic modification of nanoparticles is shown to be effective for fabricating highly concentrated nanohybrid polymers or nano inks. Furthermore, these synthesized facet-controlled nanoparticles show high catalytic activities. Indeed, it is demonstrated that steam reforming of heavy oils or biomass wastes (black liquor) can occur in water even at low temperatures.

Nucleation Mechanism of Nano-Iron Oxide in a Supercritical Water Reaction

Despite its ubiquitous application in various branches of engineering and materials science, nucleation with reactions from supercritical water remains elusive. In this work, molecular dynamics with a reactive force field was studied to demonstrate the nucleation process of nano-iron oxide crystals synthesized via the supercritical hydrothermal synthesis method using Fe2(SO4)3 as the precursor. It was shown that during the early stages of nucleation, high local density areas first occurred, making these sites primary candidates for nucleation. Furthermore, the amorphous intermediate reorganizes from the aggregation and coalescence of the prenucleation clusters (PNCs) and precedes the emergence of a crystalline nucleus rather than direct one-step nucleation from classical theory. Through calculation, it was found that the average nucleation rates increase generally with the increase in reaction temperature and system density, and the volumes of nano-iron oxide clusters are maintained at 10–12 nm3 when simulating for 3 ns. It was further confirmed that increasing the reaction temperature and density is conducive to enhancing the crystallinity of iron oxide nanocrystals within a certain range of increments, with larger clusters forming faster at higher temperatures and densities. It was demonstrated that the screw dislocation formation mechanism caused nano-iron oxide lattice defects. Although not all of the nano-iron oxide nuclei generated have lattice defects, all of the perfect nano-iron oxide nuclei are generated by adjusting the configuration of nano-iron oxide nuclei with lattice defects.

Supercritical Hydrothermal Synthesis of Nano-Zirconia: Reaction Path and Crystallization Mechanisms of Different Precursors

Nano-zirconia exhibits excellent mechanical properties (fracture toughness, hardness and flexural strength), thermal performance (thermal conductivity), optical properties and biological properties for various ceramic-based applications. Supercritical hydrothermal synthesis technology is an advanced and environmental-friendly method for preparing nanoscale powders. In this paper, the formation and crystallization of nano-zirconia during supercritical hydrothermal synthesis were systematically investigated. Different precursors (zirconium nitrate, zirconium oxychloride, zirconium acetate) reacted in supercritical water and the quality of nano-zirconia in different systems were comprehensively evaluated using XRD, TEM, IR, BET analysis. Finally, the most suitable precursor type was identified and the reaction path as well as crystallization mechanisms of different precursors were revealed.

Color-controlled nonstoichiometric spinel-type cobalt gallate nanopigments prepared by supercritical hydrothermal synthesis.

Spinel-type inorganic pigments with intensive color and chemical/thermal stability are showing extensive applications that could be further broadened by color manipulation and improvement of the material properties through nanosizing. In this study, we report the supercritical hydrothermal synthesis of nonstoichiometric spinel-type cobalt gallate nanoparticles (Co-Ga NPs) with controlled color. Without the conventional calcination procedure, NPs with greenish-blue, blue, and yellowish-green colors were synthesized from precursor solutions at pH 7, 9, and 11, respectively, with a low Co/Ga molar ratio of 0.25. X-ray diffraction, scanning/transmission electron microscopy, and inductively coupled plasma-atomic emission spectroscopy methods suggest that the products were spinel-type cobalt gallate NPs with high crystallinity and a nonstoichiometric composition. Based on an X-ray absorption fine structure investigation, the prepared nonstoichiometric Co-Ga NPs were found to have different cationic configurations from stoichiometric CoGa2O4 produced by a solid-state reaction during calcination. Meanwhile, the degrees of distortions at tetrahedral and octahedral sites in the NPs were evaluated by Raman spectroscopy. In particular, nonstoichiometric Co-Ga NPs with a blue color were prepared without calcination for the first time and were found to have lower tetrahedral cobalt occupancy but comparable octahedral cobalt occupancy and larger polyhedral distortions at tetrahedral sites when compared to calcined CoGa2O4. We also discuss strategies that could realize Co-Ga NPs with a more brilliant blue color using the present technique based on an investigation of the growth process.

Investigation of the interaction between adsorbed water and various morphologies of boehmite nanoparticles prepared by continuous supercritical hydrothermal synthesis

Boehmite applications are often subject to specific characteristics in terms of morphologies and surface properties. The versatile continuous supercritical hydrothermal method allows synthesizing various boehmite morphologies from rhombic to hexagonal platelet-like nanoparticles depending on the preferential adsorption of part of the precursor after transformation species on the (020) surface resulting in a preferential growth along [001] and mostly [100] directions. This adsorption on surface free sites plays the role of morphology stabilizer and results in the reduction of adsorbed water. The interaction between adsorbed water and the boehmite morphology depends on various parameters: 1) the structure order of boehmite as exhibited by XPS analysis associated with 1H Solid-State NMR spectroscopy and, 2) the number of available free sites, which tend to be filled by the adsorption of nitrates as observed by Raman spectroscopy.

From modification to mechanism: Supercritical hydrothermal synthesis of nano-zirconia

Nano-zirconia has been widely applied due to its excellent physical and chemical properties (e.g., high strength, corrosion resistance, oxygen ion conductivity). Existing preparation methods of nano-zirconia tend to require long reaction time, and the sizes of final particles are large with uneven distributions. Sub-/supercritical hydrothermal synthesis of nanoparticles is favored by researchers owing to controllable reaction process, uniform particle size distribution, good reproducibility, short reaction time, high conversion rate and harmlessness to environment. In this paper, the characteristics and mechanisms of dissolution, crystallization and growth of nano-zirconia during sub-/supercritical hydrothermal synthesis are systematically reviewed. The influences of process and material parameters on the size and purity of particles are analyzed. Then, the reaction mechanism and product phase transition mechanism during hydrothermal synthesis of zirconia are summarized to provide a theoretical reference for the oriented preparation. Finally, the improvement and commercialization of sub-/supercritical hydrothermal synthesis technology are evaluated, and the future research topics are proposed.

Supercritical hydrothermal synthesis of silver nanoparticles, composites, and their characterizations

Silver nanoparticles (AgNPs) and silver nanoparticles doped activated carbon (AC-Ag) composite materials were synthesized by hydrothermal processes in supercritical water conditions (29 MPa and 400 °C) using batch reactor. We studied the influence of the precursor solution concentration, reaction temperature under the hydrothermal conditions, and synthesis time on the properties of synthesized materials. The properties of plain AgNPs and AC-Ag composite materials synthesized in supercritical water, including crystallinity, particle size, and molecular interactions between AC and Ag were investigated, comprehensively. Compared to the plain AgNPs, the activated carbon-supported Ag nanocomposite was synthesized faster due to the active functional groups of activated carbon. Furthermore, the FTIR results reveal that the silver nanoparticles are attached to the activated carbon surface in the presence of oxygen bonded carbonyl and carboxyl groups. The nano-sized metal silver particles were observed on the AC surface when analyzed by TEM and XRD. All results imply that the supercritical water condition allows the formation of silver particles less than 100 nm either in the form of plain particles or deposited on the activated carbon surface using the silver acetate precursor solution. This environmentally benign supercritical hydrothermal process can replace the conventional method and become a novel synthesis method for preparing various new materials.

Supercritical hydrothermal synthesis of nano-ZrO2: Influence of technological parameters and mechanism

Supercritical hydrothermal synthesis is a promising technology to produce nano-ZrO2 due to its simple craft, short reaction time and low cost. In this paper, nano-ZrO2 particles with an average particle size of 5.66–35.43 nm were successfully prepared by supercritical hydrothermal method. Meanwhile, the influences of reaction temperature, pressure, time, precursor concentration and pH value on the particle size distributions and surface topographies of nano-ZrO2 were investigated under different conditions. Reaction time and precursor concentration were strong influencing factors of particle sizes and distributions. The reaction mechanisms of different crystal forms were controlled by pH value of the precursor. Under acidic conditions, soluble [Zr4(OH)8(H2O)16] 8+ was produced due to the hydrolysis of precursors, and the appearance of abundant monoclinic nucleus was triggered subsequent condensation of [Zr4(OH)8(H2O)16] 8+. In-situ crystallization dominated the crystallization process and the tetragonal phase was formed by the rearrangement of the precursor structures. The intermediate product Zr(OH)3NO3 was detected under high pH conditions.

Local and long-range atomic/magnetic structure of non-stoichiometric spinel iron oxide nanocrystallites.

Spinel iron oxide nanoparticles of different mean sizes in the range 10-25 nm have been prepared by surfactant-free up-scalable near- and super-critical hydro-thermal synthesis pathways and characterized using a wide range of advanced structural characterization methods to provide a highly detailed structural description. The atomic structure is examined by combined Rietveld analysis of synchrotron powder X-ray diffraction (PXRD) data and time-of-flight neutron powder-diffraction (NPD) data. The local atomic ordering is further analysed by pair distribution function (PDF) analysis of both X-ray and neutron total-scattering data. It is observed that a non-stoichiometric structural model based on a tetragonal γ-Fe2O3 phase with vacancy ordering in the structure (space group P43212) yields the best fit to the PXRD and total-scattering data. Detailed peak-profile analysis reveals a shorter coherence length for the superstructure, which may be attributed to the vacancy-ordered domains being smaller than the size of the crystallites and/or the presence of anti-phase boundaries, faulting or other disorder effects. The intermediate stoichiometry between that of γ-Fe2O3 and Fe3O4 is confirmed by refinement of the Fe/O stoichiometry in the scattering data and quantitative analysis of Mössbauer spectra. The structural characterization is complemented by nano/micro-structural analysis using transmission electron microscopy (TEM), elemental mapping using scanning TEM, energy-dispersive X-ray spectroscopy and the measurement of macroscopic magnetic properties using vibrating sample magnetometry. Notably, no evidence is found of a Fe3O4/γ-Fe2O3 core-shell nanostructure being present, which had previously been suggested for non-stoichiometric spinel iron oxide nanoparticles. Finally, the study is concluded using the magnetic PDF (mPDF) method to model the neutron total-scattering data and determine the local magnetic ordering and magnetic domain sizes in the iron oxide nanoparticles. The mPDF data analysis reveals ferrimagnetic collinear ordering of the spins in the structure and the magnetic domain sizes to be ∼60-70% of the total nanoparticle sizes. The present study is the first in which mPDF analysis has been applied to magnetic nanoparticles, establishing a successful precedent for future studies of magnetic nanoparticles using this technique.