Supercritical water can provide an excellent reaction environment for hydrothermal crystallization of metal oxide particles. Because of the drastic change of properties of water around
the critical point, density, dielectric constant, and ionic product, the phase behavior for the
supercritical water−light gas (O2, H2, etc.) system and reaction equilibrium/rate can be varied
to synthesize new materials or define particle morphologies. In this work, hydrothermal
crystallization experiments were performed with several types of flow apparatuses that allow
convenient manipulation of variables such as temperature, pressure, and residence time. The
proposed supercritical hydrothermal synthesis method has the following desirable features: (1)
ultrafine particles can be produced, (2) morphology of the produced particles can be controlled
with small changes in pressure or temperature, and (3) a reducing or oxidizing atmosphere can
be applied by introducing oxygen, hydrogen, or other gases. An overview of this method is given
for functional material synthesis of significant industrial interest including barium hexaferrite
magnetic particles, YAG/Tb phosphor fine particles, and lithium cobalt fine crystals.
This paper summarizes specific features of supercritical hydrothermal synthesis of metal oxide particles. Supercritical water allows control of the crystal phase, morphology, and particle size since the solvent's properties, such as density of water, can be varied with temperature and pressure, both of which can affect the supersaturation and nucleation. In this review, we describe the advantages of fine particle formation using supercritical water and describe which future tasks need to be solved.
Prussian blue is a historical pigment synthesized for the first time at the beginning of 18th century. Here we demonstrate that the historical pigment exhibits surprising adsorption properties of gaseous ammonia. Prussian blue shows 12.5 mmol/g of ammonia capacity at 0.1 MPa, whereas standard ammonia adsorbents show only 5.08-11.3 mmol/g. Dense adsorption was also observed for trace contamination in atmosphere. Results also show higher adsorption by Prussian blue analogues with the optimization of chemical composition. The respective capacities of cobalt hexacyanocobaltate (CoHCC) and copper hexacyanoferrate (CuHCF) were raised to 21.9 and 20.2 mmol/g, the highest value among the recyclable adsorbents. Also, CoHCC showed repeated adsorption in vacuum. CuHCF showed regeneration by acid washing. The chemical state of the adsorbed ammonia depends on the presence of the water in atmosphere: NH3, which was stored as in the dehydrated case, was converted into NH4(+) in the hydrated case.
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