Atomic‐scale analysis of the cation valence state distribution will help to understand intrinsic features of oxygen vacancies (VO) inside metal oxide nanocrystals, which, however, remains a great ...challenge. In this work, the distribution of cerium valence states across the ultrafine CeO2 nanocubes (NCs) perpendicular to the {100} exposed facet is investigated layer‐by‐layer using state‐of‐the‐art scanning transmission electron microscopy‐electron energy loss spectroscopy. The effect of size on the distribution of Ce valence states inside CeO2 NCs is demonstrated as the size changed from 11.8 to 5.4 nm, showing that a large number of Ce3+ cations exist not only in the surface layers, but also in the center layers of smaller CeO2 NCs, which is in contrast to those in larger NCs. Combining with the atomic‐scale analysis of the local structure inside the CeO2 NCs and theoretical calculation on the VO forming energy, the mechanism of size effect on the Ce valence states distribution and lattice expansion are elaborated: nano‐size effect induces the overall lattice expansion as the size decreased to ≈5 nm; the expanded lattice facilitates the formation of VO due to the lower formation energy required for the smaller size, which, in principle, provides a fundamental understanding of the formation and distribution of Ce3+ inside ultrafine CeO2 NCs.
Atomic‐scale cerium valence state distribution layer‐by‐layer in the surfactant‐modified CeO2 nanocubes is achieved using scanning transmission electron microscopy and electron energy loss spectroscopy. It is demonstrated that the increasing amount of Ce3+ in the center layers of CeO2 nanocube (as the size reduced to approximately 5 nm) is related to the nanosize‐effect induced local lattice expansion.
A continuous flow reaction process in which a metal salt solution is rapidly mixed with high-temperature water was employed to achieve rapid heating up to supercritical conditions. A quarter of a ...century has passed since the supercritical hydrothermal method was first proposed. This paper introduces recent advances in science and technology related to the supercritical process. Process design, kinetics, reaction atmosphere (redox) control, morphology control, organic modification of particles, nanocatalysts, and organic-inorganic hybrid materials are reviewed for promising applications of the supercritical process.
Improving the potential of promising CeO2-based nanocatalysts in practical applications requires an atomic-scale analysis of the effects of active dopants on the distribution of Ce valence states and ...the formation of oxygen vacancies (VOs). In this study, a Cr dopant is introduced into the cubic {100}-faceted CeO2 nanocrystals (NCs) with an average size of 7.8 nm via supercritical water. The Cr dopants substitute Ce sites in the amount of approximately 3 mol%. Based on the aberration-corrected STEM-EELS, the effects of Cr dopant on the distribution of cerium valence states are investigated layer by layer across the ultrafine Cr-substituted CeO2 NC perpendicular to the {100} exposed facet. It is found that an increased amount of Ce3+ cations is present in Cr-substituted CeO2 NCs, particularly in the internal atomic layers, compared to the pristine CeO2 NCs. The atomic-scale analysis of the local structure combined with theoretical calculations demonstrates that Cr dopant reduces the formation energy of VOs and increases the mobility of oxygen atoms for the nano-sized CeO2. These effects, in principle, result in an improved oxygen storage capacity and provide a fundamental understanding of role of the dopant in the formation and distribution of VOs in the doped CeO2 NCs.
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Two techniques using subcritical and supercritical water are introduced herein: supercritical hydrothermal synthesis of metal oxide nanoparticles and hydrothermal processing of waste and biomass. ...Recent progress of supercritical hydrothermal synthesis method is reviewed particularly for oxidation catalysts. Then hydrothermal treatment methods for biomass and waste are summarized from the viewpoint of phase behavior, homogeneous reaction, and reaction with catalysts. Fractionation of valuable materials through reforming of biomass and waste using the characteristic properties of the hydrothermal reaction field is emphasized. Cellulose and lignin, the major components of woody materials, and heavy oil as a representative waste material have been studied intensively for treatment under hydrothermal conditions. Process design aspects are discussed to provide a future scope of related technologies.
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In this study, the 3-phenylpropionic acid (3-PPA)-modified Bi
2
O
3
nanoparticle-loaded plastic scintillators were synthesized to obtain fast scintillators having high detection efficiency of ...high-energy X-rays. To reach a high light yield, the content of 2-(4-tert-butylphenyl)-5-(4-phenylphenyl))-1,3,4-oxadiazole (b-PBD) in the plastic scintillators was optimized. The detection efficiency for high-energy photons was enhanced by the incorporation of surface-modified Bi
2
O
3
nanoparticles of less than 10 nm into scintillators at 5 or 10 wt%. In the pulse-height spectra, the photoelectric peak positions were located at the highest channels for the samples containing 0.50 or 1.0 mol% b-PBD regardless of the Bi concentration. The photoelectric peak positions shifted to lower channels with a further increase in the b-PBD concentration, which indicates that the light yield decreased due to concentration quenching. In addition, the time resolution of the detector equipped with the studied samples was of the sub-nanosecond scale, suggesting that they had a very fast response.
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•Fractionation of cellulose and lignin was studied.•Hydrolysis rate of cellulose was accelerated by cellulose dissolution.•Retro-aldol reaction products from glucose increased at high ...T and low P.•Cresol captured aldehydes and suppressed the polymerization of phenolic compounds.•{001} Surface exposed CeO2 oxidized aldehydes and suppressed the polymerization.
This paper has two parts: The first part covers the review of previous researches on biomass fractionation based on mechanistic and kinetic viewpoints. Since the major components of biomass are cellulose and lignin, fractionation of each component is discussed. Kinetics and reaction mechanism of cellulose hydrolysis, glucose fractionation, and decomposition are also summarized. Based on the results of these basic studies, new processes of aldehyde recovery from cellulose/glucose and cresol addition method for fractionation from lignin are introduced. The second part of this paper proposes a new approach (original research) to decompose lignin and biomass with CeO2 nanocatalysts fabricated by supercritical hydrothermal synthesis method. Char formation decreased, and liquid product yield increased when the nanocatalyst, {001} surface exposed cubic CeO2 was used. This is probably because of the suppression of Friedel-Crafts reaction due to the oxidation of aldehydes, which could be the bridge molecules of phenolic structures.
To determine whether neutrinoless double-beta decay occurs is an enormous challenge in particle physics. For this purpose, developing a highly transparent liquid scintillator that contains a ...candidate isotope at high concentration is required. In this work, 100Mo was selected as the isotope and surface-modified nanoparticles were applied. AMoO4 alkaline earth molybdates (A = Ca, Sr, and Ba) were synthesized with a subcritical hydrothermal method so that they were well dispersed in organic solvents. The crystalline phase of the nanoparticles was confirmed with X-ray diffraction measurements. The particle size of SrMoO4 nanoparticles was found to be the smallest from a transmission electron microscope examination. The SrMoO4 nanoparticles were incorporated in liquid scintillators, resulting in high transparency and efficient scintillation. In conclusion, liquid scintillators loaded with organic modified SrMoO4 nanoparticles were successfully developed.
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•New Invention comes out through algebraic operation of previous inventions.•Use of SCF is effective to control interactions during nano structure fabrication.•To establish “unit ...operation of SCF process” is the key to develop SCF technologies.
Nanomaterials exhibit size- and/or structure-dependent properties, which must be controlled. Controlling the interaction between the component materials and medium is important in the formation of structures. This is particularly true for nanomaterials because their interaction energy is extremely large. Controlling the interaction between nanomaterials and the medium is crucial, together with the surface design. Supercritical fluids can freely control the physical properties and phase behavior, thus tuning the interaction between nanomaterials. Therefore, supercritical fluid technology should be used for the synthesis of nanoparticles, surface design, and formation of nanostructures, thus solving the problems associated with conventional processes. For the rational design of nanomaterials and synthetic process, a holistic understanding of the phase equilibria, solvent effect on kinetics, interaction between nanomaterials and the fluid, and the transport phenomena in supercritical fluids is necessary. This approach will help establish the “unit operation” of supercritical fluids. The use of supercritical fluids can also be an effective approach for understanding the mechanism of formation of nanostructures. Through fusion with other categories of science, such an approach could pave the way for establishing a holistic nanomaterials science.
In this study, supercritical hydrothermal synthesis of BaZrO
3
and its formation mechanism during the synthesis were studied using a continuous flow reactor. The Mono-phase, nano-sized BaZrO
3
was ...successfully synthesized at a temperature of 400 °C and a pressure of 30 MPa using oxy-zirconium nitrate and excess barium hydroxide as the starting materials. The formation mechanism of BaZrO
3
was studied by examining the time dependence of size and composition using XRD, TEM, and ICP. As a result of the time-resolved experiment, the following formation mechanism was revealed. At the first stage of the reaction (~0.1 s), a perovskite structure forms though it has many defects of Ba site. The particle size increases to 20 nm range by coalescence at the middle stage (~1 s) and becomes constant at the last stage (1–10 s). Ba site defects are filled by the uptake of Ba with increasing time until the last stage (~10 s). The elucidated formation mechanism, i.e., the coalescence of nuclei and uptake of Ba, is significant to develop a new methodology for controlling the size and composition of the BaZrO
3
nanoparticles.
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