Defects at the (1 1 0) surface of rutile TiO2 from ab initio calculations

Defect segregation of oxygen vacancies, protons, titanium vacancies, and the mutual complex between the two latter (i.e. Ruetschi type defects) at the (1 1 0) surface of rutile TiO₂ has been studied through ab initio Density Functional Theory calculations (DFT). Oxygen vacancies and protons form mainly on the 2-fold coordinated oxygen sites, while titanium vacancy related defects are expected to be most prominent at the 5-fold titanium sites. All included defects are shown to be more stable at the surface than in the bulk oxide, and the relative tendency to segregate is most pronounced for oxygen vacancies. Based on the obtained defect segregation energies and previously calculated bulk defect chemistry at lower temperatures, our estimated surface defect formation energies indicate that the surface of TiO₂ can be expected to be dominated by titanium vacancies, protons and their mutual complexes at lower temperatures under wet, oxidizing conditions. Finally, possible routes of dissociative water adsorption on the surface, involving the aforementioned defects are discussed.     

In situ metal doping during modified anodization synthesis of Nb2O5 with enhanced photoelectrochemical water splitting

A new technique of in situ doping of alkali metal (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) in Nb₂O₅ was showcased by the modified anodization of Nb foils at high frequency, negative‐to‐positive pulsed voltage. At the optimized dopant concentration and synthesis condition, the doped‐Nb₂O₅ shows twofold enhancement in photoelectrochemical water splitting efficiencies compared with the undoped Nb₂O₅ electrode, as a result of improved charge carrier density and enhanced surface charge transfer. © 2015 American Institute of Chemical Engineers AIChE J, 62: 352–358, 2016     

Formation mechanisms of insoluble Cs particles observed in Kanto district four days after Fukushima Daiichi NPP accident

ABSTRACT

Insoluble Cs particles (Type A) were first observed in Tsukuba city in the morning of 15 March 2011. The particles were considered to have been formed in the reactor pressure vessel of Unit 2 by evaporation/condensation based on the measured ¹³⁴Cs/¹³⁷Cs ratio and the accident progression of each Unit. However, the particles were covered by almost pure silicate glass and showed a trace of quenching. This can be explained by other mechanisms, that is, the particles were formed by the melting of glass fibers of the high efficiency particulate air filter in the Standby Gas Treatment System owing to the fire due to hydrogen detonation in Unit 3 at 11:01 on March 14 and atomization due to the explosion, followed by quenching of the molten materials. Although the particles formed in this way were mostly dispersed to the sea by wind at that time, some of them were deposited on the lower elevation of the reactor building of Unit 3, and they could have been resuspended and released into the environment, by the flow owing to the generation of a large amount of steam as result of a restart of core cooling water injection at 02:30 on March 15.     

On the structural origin of the catalytic properties of inherently strained ultrasmall decahedral gold nanoparticles

A new mechanism for reactivity of multiply twinned gold nanoparticles resulting from their inherently strained structure provides a further explanation of the surprising catalytic activity of small gold nanoparticles. Atomic defect structural studies of surface strains and quantitative analysis of atomic column displacements in the decahedral structure observed by aberration corrected transmission electron microscopy reveal an average expansion of surface nearest neighbor distances of 5.6%, with many strained by more than 10%. Density functional theory calculations of the resulting modified gold d-band states predict significantly enhanced activity for carbon monoxide oxidation. The new insights have important implications for the applications of nanoparticles in chemical process technology, including for heterogeneous catalysis.     

Flame preparation of visible-light-responsive BiVO4 oxygen evolution photocatalysts with subsequent activation via aqueous route

Visible-light-active BiVO(4) photocatalyst prepared by a one-step flame spray pyrolysis demonstrates the structural evolution from amorphous to crystalline scheelite-tetragonal and further to scheelite-monoclinic (the photocatalytic active phase). Up to 95% scheelite-monoclinic content, the rest being scheelite-tetragonal, can be achieved in situ by exposing the collection filter to higher flame temperature. Reasonable activity in terms of photocatalytic O(2) evolution was obtained with the increase in crystallinity and scheelite-monoclinic content. Although analogous postcalcination of BiVO(4) improves crystallization and phase transformation, it inevitably induces vacancy defects that are detrimental to the photocatalytic activity. Hence a facile aqueous acid treatment on the flame-made BiVO(4) is introduced, which in the presence of small addition of Bi and V promotes full transformation to scheelite-monoclinic and reduces charge trapping defects. As a result, the photocatalytic O(2) evolution activity was increased by a remarkable 5 folds compared to the best performing untreated flame-made BiVO(4).     

Chemical Expansion and Change in Lattice Constant of Y‐Doped BaZrO3 by Hydration/Dehydration Reaction and Final Heat‐Treating Temperature

Knowledge of thermal behavior of electrolyte is important for fuel cell fabrication. In this study, using high‐temperature X‐ray diffraction analysis (HT‐XRD) and thermo‐mechanical analysis (TMA), a systematic investigation of lattice constants was performed on Y‐doped BaZrO₃, which is a promising candidate for electrolyte in protonic ceramic fuel cells. The results revealed that a chemical expansion was observed between 300°C and 450°C during the heating process in HT‐XRD, and was attributed to the dehydration of BZY. Furthermore, it was found that the lattice constants of the samples doped with Y, Sm, Eu, and Dy were larger for the ones finally heat‐treated at 1600°C for sintering than those heat‐treated at 1300°C for synthesizing. The similar behavior was not observed in Sc‐doped samples.     

Position and speed sensorless control system of permanent magnet synchronous motor with parameter identification

The model parameters of a permanent magnet synchronous motor (PMSM) are required for high‐performance control and a model‐based sensorless control. This paper proposes a sensorless control system of PMSM that does not need parametric information beforehand. The parameters of a PMSM drive system, including the inverter, are identified at standstill and under operating conditions. At first, the initial rotor position is estimated by a signal injection sensorless scheme in which the machine parameters are not required. After the initial position has been estimated, the resistance, including the on resistance of the IGBT, the voltage error caused by dead time in the inverter, and d‐axis and q‐axis inductances, are identified at standstill. After the motor starts by the signal injection sensorless control, the sensorless scheme changes to an extended EMF estimation scheme. The estimated parameters for the resistance, and daxis and q‐axis inductances are used in such sensorless control. The magnet flux‐linkage, which cannot be estimated at standstill, is identified under the sensorless operation at medium and high speeds. The effectiveness of the proposed method is verified by several experimental results. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 160(2): 68–76, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/ eej.20548     

Influence of thermal properties of zirconia shroud on analysis of PHEBUS FPT0 bundle degradation test with ICARE2 code

In the FPT0 test of the PHEBUS/FP program, it was observed that the fraction of liquefied UO₂ reached 50%, which is much larger than the expected maximum value of 20%. Most of the post-test analyses with various computer codes underpredicted the bundle temperature during a late phase and could not reproduce such a large core degradation. In most of the previous analyses, the shroud thermal conductivity evaluated based on the Pears' ZrO₂ specific heat data and the thermal diffusivity measured by JAERI was used. However, recent thermal property data books adopt a lower specific heat than measured by Coughlin and King's at high temperature. The present analyses with ICARE2 showed that the FPT0 bundle behavior could be mostly reproduced by using the shroud thermal conductivity based on Coughlin and King's. If the present calculation is assumed to be correct enough, the shroud thermal conductivity at high temperature could be smaller than the current evaluation based on the Pears' data. Since the shroud thermal conductivity has thus a strong effect on the bundle behavior, further measurement and evaluation of the thermal properties of the shroud are highly recommended.     

Nanosized Au Particles as an Efficient Cocatalyst for Photocatalytic Overall Water Splitting

The effect of the photodeposition of gold particles onto several photocatalysts on the photocatalytic activities was studied. The photocatalytic activities of K₄Nb₆O₁₇, Sr₂Nb₂O₇, KTaO₃ NaTaO₃, and NaTaO₃ doped with La for water splitting were improved when gold particles were deposited. The latter were nanoparticles, consistent with their surface plasmon absorption. The nanosized gold particle functioned as an efficient cocatalyst for photocatalytic water splitting by assisting H₂ evolution.