Continuous supercritical hydrothermal synthesis of controlled size and highly crystalline anatase TiO2 nanoparticles

The production of size-controlled and highly crystalline anatase titanium dioxide (TiO₂) nanoparticles was carried out under supercritical hydrothermal conditions (400 °C and 30 MPa) in a continuous flow apparatus with a residence time of 1.7 s. An industrially useful titanium sulfate (Ti(SO₄)₂) solution was used as the starting solution. KOH was used to change TiO₂ solubility and pH and thereby control the particle size. The apparatus comprised two micromixers operating at high temperature. The first mixer was configured to prepare a supercritical aqueous KOH solution from supercritical water (SC-H₂O) and KOH. The second mixer combined this KOH solution with aqueous Ti(SO₄)₂. In situ pH control and homogeneous nucleation were achieved in the second mixer. This two-step high-temperature micromixing process produced reasonably small and homogeneous particles. The particles were characterized by transmission electron microscopy (TEM) on the basis of morphology, average size, and size distribution, together with the coefficient of variation (CV). Powder X-ray diffraction (XRD) was used to determine the crystal structure and crystalline size. The weight loss of material was found through thermogravimetric (TG) measurement. The crystal structure of the product was assigned to the anatase single phase. The average particle size could be adjusted in the range 13–30 nm while maintaining a CV of 0.5 by changing the KOH concentration. At low pH, the powder XRD results for crystallite size were in good agreement with the average particle size measured by TEM, confirming that the products were single crystals of TiO₂ nanoparticles. When the reactor temperature was increased from 400 to 500 °C, the weight loss decreased from 4.5 to 2.5%, keeping the average particle size and high crystallinity of the TiO₂ particles unchanged.     

Particle diameter prediction in supercritical nanoparticle synthesis using three-dimensional CFD simulations. Validation for anatase titanium dioxide production

A mathematical model is proposed and validated with experimental data for the estimation of the average diameter attained by the particles that are generated by decomposition of an organometallic precursor in supercritical CO₂ (scCO₂). Experiments have been performed for the synthesis of TiO₂ anatase nanoparticles, using diisopropoxititanium bis(acetilacetonate) (DIPBAT) as precursor, ethanol as reactant and scCO₂ as solvent. The model is solved by using computational fluid dynamics (CFD) for the experimental geometry: a tee piece as mixer followed by a cylindrical reactor. Peng-Robinson EOS with Huron-Vidal mixing rule has been used to predict density variations within hydrodynamic equations. A pseudo-first order kinetic (r_TiO2=kC_DIPBAT, with ) has been proposed for mass and population balances. The CFD simulations predict a reduction on particle diameter from 400 to 200 nm when the Reynolds number is increased from 280 to 1500. In this range, deviations with experimental data are lower than 15%. A parametric study of the kinetic constant reveals that for faster reactions (Da?10^-3) the trend of particle size with the Reynolds number is inverse, and particle diameter increases when the Reynolds number is increased.     

超临界水热合成制备纳米微粒材料

简述了超临界水热合成法制备纳米微粒材料技术的微粒形成机理、过程基本工艺及目前所取得的研究成果。讨论了未来需要解决的问题。     

Hydrothermal synthesis and in situ surface modification of boehmite nanoparticles in supercritical water

In situ surface modification of boehmite (AlOOH) nanoparticles during hydrothermal synthesis in supercritical water was examined by adding CH₃(CH₂)₄CHO and CH₃(CH₂)₅NH₂ as modifier reagents to the reactants. Changes in surface properties of the nanoparticles by surface modification was observed by FTIR, dispersion in solvents and TEM analyses, which demonstrated that reagents chemically binded onto the surface of the AlOOH nanoparticles. The results of SEM and TEM pictures show that the surface modification affects crystal growth and reduces the particle size and changes the morphology of the particles.     

Continuous supercritical hydrothermal synthesis of dispersible zero-valent copper nanoparticles for ink applications in printed electronics

Surface-modified zero-valent copper nanoparticles (CuNPs) are of interest as conductive inks for applications in printed electronics. In this work, we report on the synthesis, stability and characterization of CuNPs formed with a continuous supercritical hydrothermal synthesis method. The precursor, copper formate, was fed as an aqueous solution with polyvinylpyrrolidone (PVP) surface modifier and mixed with an aqueous water and formic acid stream to have reaction conditions of 400 °C, 30 MPa and 1.1 s mean residence time. The reaction pathway seemed to proceed step-wise as the hydrolysis of copper formate, followed by dehydration to oxide products and subsequent reduction by hydrogen derived from precursor and formic acid decomposition. The formed surface-modified zero-valent CuNPs had particle sizes of ca. 18 nm, were spherical in shape and contained no oxide contaminants. The formed CuNPs were found to exhibit long-term (>1 year) stability in ethanol as evaluated by shifts in the surface plasmon resonance band of product solutions. Conductive films (0.33 μm thickness) prepared with the CuNPs had a resistivity of 16 μΩ cm. The methods reported in this work show promise for producing conductive inks for use in practical printed electronics.     

Comparative hydrothermal synthesis of hydroxyapatite by using cetyltrimethylammonium bromide and hexamethylenetetramine as additives

Comparative synthesis ways for preparing HA (Ca₁₀(PO₄)₆(OH)₂) nanoparticles in presence of hexamethylenetetramine (HMTA) and cetyltrimethylammonium bromide (CTAB) were carried out. The reactions were performed in a Teflon-lined stainless-steel reactor at 120 °C during 12 h. The effects of the additive concentration and the cooling mode (fast and slow) were analyzed. The obtained powders were characterized by X-ray Diffraction (XRD), Raman Spectroscopy, Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM). The two hydrothermal ways carried out for preparing HA nano powders produced a pure crystalline phase of HA. When the fast cooling mode was used, the obtained particles exhibited smaller mean particle sizes. The highest concentrations of used additives (HTMA or CTAB) resulted in opposite effect on the obtained mean particle size of HA particles. These observations were associated to the different behavior of these additives in the HA formation processes.     

Effect of KOH on the continuous synthesis of cobalt oxide and manganese oxide nanoparticles in supercritical water

The effects of KOH on the supercritical hydrothermal synthesis of cobalt oxide and manganese oxide particles are investigated using a continuous-flow reactor. Significant changes in morphology, particle size, and oxidation state are observed by adding KOH. The spinel Co₃O₄ phase is transformed to a rocksalt CoO phase and the pyrolusite MnO₂ phase is transformed to a hausmannite Mn₃O₄ phase in the presence of 0.5 M KOH. The average particle size of the metal oxides decreased with an addition of KOH. The OH⁻ ions of KOH may act as a reducing agent as well as a supersaturation enhancing agent under supercritical water conditions.     

Design of a reactor operating in supercritical water conditions using CFD simulations. Examples of synthesized nanomaterials

Direct information about fluids under supercritical water conditions is unfeasible due to the engineering restrictions at high pressure and high temperature. Numerical investigations based on computational fluid dynamics (CFD) calculations are widely used in order to get extensive information on the fluid behavior, particularly to help the design of a new reactor. This paper presents the numerical investigations performed on an original supercritical water device, especially in the level of the reactor. CFD calculations allow to design and optimize the present reactor described in this study. Currently, this process produces some nanometric oxide powders in continuous way with a production rate of 10–15 g h⁻¹. Examples of synthesized nanomaterials are presented in order to prove the process efficiency. Crystalline ZrO₂ and TiO₂ were produced from a metallic salt and an organometallic as precursors, respectively. XRD and HRTEM analyses show nanosized particles with an uniform size distribution (≤7 nm) and a high crystallinity.     

Imaging the continuous hydrothermal flow synthesis of nanoparticulate CeO2 at different supercritical water temperatures using in situ angle-dispersive diffraction

In situ high-energy synchrotron X-ray diffraction, a non-destructive synchrotron-based technique was employed to probe inside the steel tubing of a continuous hydrothermal flow synthesis (CHFS) mixer to spatially map, for the first time, the superheated water crystallisation of nanocrystalline ceria (CeO₂) at three different (superheated-water) temperatures representing three unique chemical environments within the reactor. Rapid hydrothermal co-precipitation at the three selected temperatures led to similarly sized ceria nanoparticles ranging from 3 to 7 nm. 2D maps of CeO₂ formation were constructed from the intensity and corresponding full width at half maximum (FWHM) values of the two most intense ceria reflections (111) and (002) for all three water inlet temperatures (350, 400 and 450 °C at 24 MPa) and subsequent changes in the particle size distribution were analysed. The accompanying high-resolution transmission electron microscopy (HRTEM) and tomographic particle size maps have confirmed that the mean ceria particle size slightly increases with temperature. This X-ray tomographic imaging study amounted to a formidable technical and engineering challenge, nevertheless one that has been met; this represents a significant achievement in imaging science, given the dynamic nature and hostile environment of a working CHFS reactor.     

Modeling and simulation of a continuous biomass hydrothermal carbonization process

Abstract

This work evaluates a continuous biomass hydrothermal carbonization process through modeling and steady state simulation using the UniSim Design process simulator. The reactive process was divided into four stages: biomass hydrolysis, intermediate compounds degradation, aromatics formation, and polymerization process, which make it possible to obtain the solid product or hydrochar. Pure biomass types and their mixtures were compared, considering hydrochar and carbon yields, H/C, and O/C ratios, and their deviation from the batch process. The results of hydrochar yield indicated that biomass with high cellulose content can perform satisfactorily in the proposed model. In addition, the possibility of carrying out the process in reactive stages together with the recirculation of liquid product, allowed a greater yield with respect to the batch process. It is concluded that the proposed model improves the characteristics of the obtained hydrochar compared to its crude biomass, achieving lower proportions of hydrogen and oxygen in the solid product.     

Hydrothermal flames in supercritical water oxidation: investigation in a pilot scale continuous reactor

Hydrothermal flames at 25 MPa supercritical water environment were investigated using a 4800 ml reaction tower, in which the sapphire windows were fitted for optical access. Down flowing hydrothermal flames were observed for oxidation of 2-propanol when the reactor was fed with inlet organic concentration higher than 2 vol% and air ratio higher than 1.8. Flame temperature, as high as 1100 °C, was measured by means of a thermocouple and the temperature was found to be strongly influenced by air ratio. Effective and stable oxidation of organics with TOC removal rate of 99.9% was achieved. Dioxins were also decomposed with a ratio higher than 99.9%, within 1 min reaction time in this reactor configuration.     

Iron oxide synthesis using a continuous hydrothermal and solvothermal system

Iron oxide synthesis via a continuous hydrothermal and solvothermal reaction were studied. In the hydrothermal synthesis, fine α-Fe₂O₃ (hematite) particles were obtained at 250–420 °C and 30 MPa. The α-Fe₂O₃ crystals were grown in sub-critical water via a dissolution and precipitation process. The growth of α-Fe₂O₃ crystals in supercritical water was suppressed due to the rather low solvent power of water. Crystalline Fe₃O₄ was obtained as the temperature was raised above the supercritical state in the solvothermal preparation. Isopropanol (IPA) was oxidized in acetone which provided a reducing atmosphere. Acetone molecule adsorption onto the Fe₃O₄ particle surface inhibited crystallite growth.     

Hydrothermal synthesis of metal nanoparticles using glycerol as a reducing agent

Metal and metal oxide nanoparticles were synthesized using supercritical water (SCW) as a reaction medium and glycerol as a reducing agent at 400 °C and 300 bar. X-ray diffraction (XRD) patterns confirmed that silver, copper and nickel nitrates were reduced to zero-valent metal nanoparticles. On the other hand, cobalt, iron and manganese nitrates were partially reduced into low-valent metal oxides. Scanning electron microscopy (SEM) images showed that the reduced metals and metal oxides were smaller than the metal oxides formed without glycerol. The difference in reduction behavior of elements is explained using their reduction potentials. Glycerol proved to be an effective reducing agent for hydrothermal applications.     

Eggplant as an appreciable bio-template for green synthesis of NiO nanoparticles: Study of physical and photocatalytic properties

In this study, we described a green, sustainable, and feasible method for synthesizing 5 nm NiO nanoparticles. Eggplant skin was chosen as an appropriate bio-template with a high potential to induce its structure into the desired metal oxide. Two approaches were used and compared to synthesize NiO bio-template: hydrothermal and sol-gel. The morphology and physical properties of the obtained NiO nanoparticles were evaluated using FESEM, TEM, XRD, BET, FT-IR, TGA, and UV–Vis analyses. All these methods confirm that the hydrothermal method is a better approach for synthesizing NiO bio-template nanoparticles than the sol-gel method. UV–Vis analysis revealed that the NiO nanoparticles produced by the hydrothermal method have a low bandgap of 2.88 eV, which is a key factor for photocatalytic applications.     

Fractional factorial design for the optimization of hydrothermal synthesis of lanthanum oxide nanoparticles under supercritical water condition

In this research, synthesis of lanthanum oxide nanoparticles using supercritical water as a reaction medium in batch type reactor was studied. The crystallographic identity and morphology of the synthesized nanoparticles were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns indicate that the well-crystallized lanthanum oxide nanocrystals can be easily obtained under the current synthetic conditions. The effect of four parameters includes temperature, reaction time; primary concentration of aqueous solution of lanthanum (III) nitrate and pH of starting solution on reaction efficiency, particle size and the BET surface area were investigated using 2⁴⁻¹ fractional factorial design. Finally, by employing a regression analysis a model based on effect of significant main variables and their binary interactions was proposed which can predict the percentage of reaction efficiency with acceptable confidence.     

Synthesis of in situ functionalized iron oxide nanoparticles presenting alkyne groups via a continuous process using near-critical and supercritical water

The preparation of iron oxide nanoparticle dispersions of varying properties (e.g. color, crystal structure, particle size distribution) in a continuous hydrothermal pilot plant operating under near-critical and supercritical conditions with the aim of producing in situ functionalized nanoparticles suitable for secondary functionalization via click chemistry is reported. The effect of varying the mixing setup, reaction temperature and the starting material (iron salt) in the presence of different carboxylic acids on the resulting nanoparticle dispersions was investigated. The stability of the clickable ligands in the harsh hydrothermal environment was also tested and the clickability of the functionalized particles was demonstrated by means of XPS and fluorescence measurements after model click reactions.     

Neutron radiography and numerical simulation of mixing behavior in a reactor for supercritical hydrothermal synthesis

We have visualized the distributions of temperature and water density in a tubular flow reactor for supercritical hydrothermal synthesis of nanoparticles by neutron radiography, and investigated the effect of reactor configuration on the mixing behavior in the reactor. Here, three types of reactors were used, and the mixing behaviors of supercritical water and room‐temperature water at a T‐junction in the reactors were observed. As a result, it was revealed that the distributions of temperature and water density in the tubular flow reactors strongly depend on their configurations by neutron radiography. In addition, numerical simulations have been carried out to investigate the flow patterns and temperature distributions in the reactor in detail using the commercial software FLUENT, and it was demonstrated that the numerical results can explain the experimental results obtained by neutron radiography well. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1168–1175, 2014     

Effect of cations and anions on properties of zinc oxide particles synthesized in supercritical water

Hydrothermal synthesis of zinc oxide fine particles from zinc salt (Zn(CH₃COO)₂, ZnSO₄, Zn(NO₃)₂) and alkali metal hydroxide (LiOH, KOH) aqueous solution was carried out with a Ti alloy batch reactor in supercritical water. Particle size synthesized in LiOH solution was relatively smaller than that in KOH. Emission spectra of the particle produced from ZnSO₄ and LiOH aqueous solution shows the highest intensity among these systems. Hydrothermal synthesis of zinc oxide fine particles from Zn(NO₃)₂ and LiOH solution was also carried out with a flow-through apparatus for continuous production and rapid heating of the starting solution to supercritical states. Nanoparticles having an average particle diameter of 16 nm was produced at 659 K and 30 MPa.