Research subject: Due to the public attention to the environmental issues as well as strict environmental regulations, eco-friendly methods for synthesis of nanoparticles have received considerable attention in recent years. Research approach: In the present study, a mixed oxide nanoparticle containing cerium and zirconium (Cex-Zr1-xO2) was fabricated the in supercritical water (SCW) medium. The synthesized nanoparticles were characterized by various techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Main results: Results demonstrated that fine nanoparticles with mean size of 13± 3 nm, with high crystallinity, and with appropriate size distribution and surface area were synthesized by SCW. Moreover, an oxygen storage capacity (OSC) as high as 1. 25 mmol O2/g was estimated for Cex-Zr1-xO2 nanoparticles through temperature programmed reduction in hydrogen (H2-TPR). According to the obtained results, the Cex-Zr1-xO2 nanoparticles could be a suitable candidate for catalysts of oxidation processes as well as three-way catalyst for control of automotive exhaust gases.

Controlling the surfaces of metal oxide nanoparticles is becoming increasingly important to develop the next generation of nanodevices. In this work, the surface modification of AlOOH nanoparticles during hydrothermal synthesis was examined by adding several modifier reagents to the reactants. The results showed that reagents successfully modified and controlled the surfaces of the AlOOH  nanoparticles.      

Fine BaTiO3 nanoparticles were obtained by hydrothermal synthesis under supercritical conditions with batch and flow type experimental methods. Mixture of barium hydroxide and titanium oxide as starting solution was treated in the supercritical water at 400°C and 30 MPa. The size of nanoparticles synthesized in the flow type experiment was smaller than that in the batch type. Rapid heating in a flow reactor is effective to synthesize smaller size and narrower particle size distribution for the BaTiO3 nanoparticles. The mechanism for this result was discussed based on the solubility of titanium oxide.

In this research, the possibility of decorating SnO2 nanoparticles on bentonite as a support in supercritical water environment has been investigated. Synthesis process of bentonite-SnO2 nanocomposite was performed in a reactor made of L316 stainless steel with a volume of 20 ml. The synthesis temperature and the duration was 480 °C and two hours, respectively. The manufactured nanocomposite was evaluated by various analyzes such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to confirm the immobilization of SnO2 nanoparticles on the support. The specific surface area, size and volume of nanocomposite cavities were determined using BET analysis. The appearance of SnO2 peaks in the XRD spectrum of the nanocomposite confirmed the successful synthesis of nanoparticles on bentonite. The immobilization of SnO2 nanoparticles with a size of less than 15 nm on bentonite plates was confirmed according to SEM and TEM images. The produced nanocomposite possessed a specific surface area of 50.36 m2/g and a pore volume of 0.13 cm/g due to the presence of bentonite in its structure, which makes it suitable for use as a catalyst in various reactions.

Like any other precipitation process, in supercritical water hydrothermal synthesis (SWHS), the need to improve product quality and minimize production cost requires understanding and optimization of Particle Size Distribution (PSD). In this work, using Population Balance Equation (PBE) containing nucleation and growth terms, the reactive precipitation of zirconia nanoparticles prepared by SWHS in the batch reactor was modeled. An optimization method using genetic algorithm function in MATLAB environment was developed to find simultaneously the kinetic parameters of nucleation and crystal growth rates, used for predicting PSD in PBE. The methodology developed evaluated kinetic parameters at comparable order of magnitudes to those presented in the literature, indicating a reasonable validation of the modeling method adopted. PSD results, however, showed a weak convergence of experimental and those predicted, suggesting that here, aggregation most likely played a considerable role in the PBE modeling of SWHS preparation of the zirconia nanoparticles.

In our current study, supercritical water impregnation (SCWI) was introduced as a unique catalyst preparation method by employing the high diffusivity property of supercritical water. The method allows nano-particles to place on support surfaces in extremely dispersed conditions. The silica-based nanocatalyst granules for this purpose were prepared by initial impregnation of highly porous silica (500 m2/g) in aqueous nitrate solutions, followed by hydrothermal decomposition of the nitrates to oxides at supercritical condition. The prepared sample prior to undergoing characterized by X-ray diffractometry, transmission electron microscopy, and nitrogen adsorption analysis (BET). Although the catalytic properties of the CuO in silica supports were not evaluated, the procedure of employing supercritical water in comparison to other routes to deposit metal oxide particles on hydrophobic surfaces inside support structures offers promise for catalyst preparation without the use of toxic or noxious solvents.