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.
Display omitted
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.
Graphic abstract
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.
Display omitted
•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.
Research and development in materials science has improved tremendously over the past few decades, resulting in benefits to the quality of life of people worldwide ....
Display omitted
•La2Zr2O7 nanoparticles were synthesised in supercritical water at 450 °C and pH 11.•Highly crystalline pyrochlore La2Zr2O7 was obtained even under such mild condition.•The ...dissolution of La(OH)3 and ZrO2 was accelerated at high T and high pH condition.•The reaction time was lower owing to increasing dissolution of the intermediates.•The environmentally benign process serves as a novel synthetic method for La2Zr2O7.
Pyrochlore La2Zr2O7 nanoparticles were synthesised in supercritical water at 450 °C under a relatively short reaction time. Compared to traditional methods, e.g. solid-state reactions, hydrothermal methods, highly crystalline pyrochlore La2Zr2O7 nanoparticles were obtained even under such mild conditions. Furthermore, the supercritical hydrothermal synthesis of La2Zr2O7 nanoparticles was highly sensitive to the solution pH employed, as La(OH)3 and ZrO2 were formed instead of La2Zr2O7 at pH 10 or lower. pH-controlled dissolution of La(OH)3 and ZrO2 thus regulated the formation and growth of La2Zr2O7. This environmentally benign supercritical hydrothermal process will serve as a novel synthetic method for the preparation of La2Zr2O7 with replacing conventional processes.
PDF
