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.
The interplay between chemical dopants and topological defects plays a crucial role in electrocatalysis of doped graphene. By systematically tuning the curvatures, thereby the density of topological ...defects, of 3D nanoporous graphene, the intrinsic correlation of topological defects with chemical doping contents and dopant configurations is revealed, shining lights into the structural and chemical origins of HER activities of graphene.
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.
Chemically doped graphene with three‐dimensional bicontinuous nanoporosity is developed for rechargeable Li‐O2 batteries. N and S doping, together with a large accessible surface area and open pore ...channels of the nanoporous graphene significantly enhance the cathodic reaction kinetics of Li‐O2 batteries for an ultrahigh rechargeable capacity of 10 400 mAh g−1 and a long cycling lifetime.
► Extraction of bitumen into supercritical water attenuated coke formation. ► A continuous hydrothermal extraction method with subcritical water was employed. ► Simultaneous upgrading of bitumen and ...suppression of coke formation was possible. ► We propose that asphaltene core can work as coke precursor as well as good solvent.
Upgrading bitumen by converting asphaltene to maltene whilst suppressing coke formation is desirable though technically difficult. Mass transfer behavior under sub- and super critical water conditions was observed in an autoclave-type reactor revealing that the dissolution of asphaltene core attenuates coke formation and promotes the formation of upgraded products. On the basis of this new information, a continuous hydrothermal extraction method using near supercritical water in a column flow reactor was tested. The results at 300°C, 3–6MPa, and water flow of 3–10g/min, showed higher degree of upgrading than in the autoclave reactor. Significant conversion of asphaltene to maltene with lighter maltene composition was obtained, while coke formation was completely suppressed. A continuous hydrothermal extraction method using a column flow reactor has thus been shown can solve the problem of incompatibility between enhancing the degree of upgrading of heavy oils and suppressing the formation of coke. In the proposed mechanism, low levels of asphaltene core extraction results in the formation of coke in the oil phase, whereas high levels of asphaltene core extraction results in the suppression of coke formation due to low concentrations of asphaltene core remaining in the oil phase, and the solvent properties that asphaltene core acquires at high concentrations in the water phase.
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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Formation of La-based perovskite compounds, LaBO3 (B: Fe, Al, Mn, Co and Ni), in supercritical water was examined using a batch-type reactor vessel. Investigations of processing conditions revealed ...that the heat treatment of the equimolar nitrate solution of La and Fe with pH 8 at 450 °C and 30 MPa for 15 min was optimal for obtaining LaFeO3 as a single phase. Analysis of the products prepared under the conditions of the lower reaction temperature or the shorter reaction time elucidated the formation mechanism of LaFeO3; it was formed from a reaction of intermediate products of LaOOH and Fe2O3 at 450 °C, and LaOOH was formed via La2(OH)5.1(NO3)0.9 at the lower temperature. Adjustment of the pH of the nitrate solution to 8 by the addition of the KOH solution was important for the effective formation of the LaOOH intermediate and for the decrease in the formation of La2(OH)5.1(NO3)0.9 or La(OH)3, which had a poor reactivity with Fe2O3 or other iron-related compound at 450 °C and 30 MPa and remained as the secondary phase in the final product. The optimum conditions were applied to the synthesis of LaAlO3, LaMnO3, LaCoO3 and LaNiO3, and it was observed that a single phase of LaAlO3 was formed, while LaMnO3 was formed with several secondary phases, and LaCoO3 and LaNiO3 could not be formed at all. The ease of the formation of the La-based perovskites was mostly related to the sum of first, second and third ionization energies of B-site cations, which may mean that it is easy to form a B ion with a + 3 valence (Fe3+, Al3+, Mn3+, Co3+ and Ni3+) in supercritical water.
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
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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
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O
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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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