[en] Microwave (MW) irradiation induces interaction of the dipole moment of polar molecules or molecular ionic aggregates with alternating electronic field and magnetic field, causing molecular-level healing which leads to homogeneous and quick thermal reactions. Thus, MW heating has recently begun to attract the attention of chemists as a new method not only for synthesizing organic compounds but also for preparing inorganic ones such as nano-sized inorganic particles, molecular sieve films, and self-assembled monolayers. Although the formation mechanism of monodispersed nanoparticles under MW irradiation has not been entirely understood, we can point out several factors important for precise control of the particle size and its distribution: the purity of starting materials, effect of a reactor wall, temperature change, heating rate, homogeneity of a solution, and a stabilizing reagent. We conduct advanced research in a variety of topics that are of current interest for microwave-assisted chemistry. The areas of researches are: (1) Preparation of nanocrystallites of metals, (2) Preparation of cadmium sulfide nanocrystallites, (3) Preparation of metal oxide nanocrystallites, (4) Chemical modification of surface of carbon black, (5) Detoxification of halogenated organics through hydrogenation catalyzed by Pt supported on carbon. Precise control of the particle size and its distribution is achieved in the preparation of nanocrystallites of inorganic compounds, only when the preparation is carried out under microwave irradiation. We propose that nucleation and its growth can be separately controlled in the chemical reactions induced by MW irradiation. It is discussed related to the advantageous characters in heating induced by MW irradiation. Detoxification of halogenated aromatic compounds, such as PCB and dioxin, is also taken as an important and urgent issue in environmental problems, to which microwave-assisted chemistry can contribute

[en] Full text: Microwave-assisted catalytic hydrogenation of chlorinated aromatics, such as 4-chlorophenol, pentachlorophenol, and polychlorinated benzene by activated carbon supported platinum (Pt/C) results in rapid dechlorination, giving chlorine-free chemicals such as phenol, cyclohexanol, benzene, and other chlorine-free highly reduced compounds, which provides a facile and complete detoxification method for robust chlorinated aromatics. Dechlorination processes of polychlorinated aromatics have been paid much attention in view of detoxification of halogen-containing highly toxic compounds, such as pentachlorophenol (PCP), dichlorodiphenyltrichloroethane (DDT), and polychlorinated biphenyl (PCB). Oxidative degradation of chlorinated chemicals, i.e., combustion or photo-oxidation into CO₂, HCl, and H₂O, faces difficulty because 1) the more halogen atoms the compounds contain, the more resistant to oxidation they are, and 2) the formation of thermally stable and toxic intermediates or byproducts such as dioxins is unavoidable in the processes. Reductive dechlorination of polychlorinated aromatics is regarded as an alternative dechlorination method to avoid the above-mentioned problems. On the other hand, microwave(MW)-assisted chemistry has rapidly blossomed in organic and inorganic syntheses, catalysis, and hazardous waste treatment due to its rapid heating ability depending on the dielectric loss of substances. Interestingly, surfaces of carbon black and related carbon powder can be heated up to 1556 K under MW irradiation in a short time (e.g., 1 min). How to take the advantage of the extremely high temperature on carbon black caused by MW rapid heating provokes our interest in efficient dechlorination of chlorinated aromatics by MW-assisted hydrogenation in the presence of activated carbon-supported platinum (Pt/C, 5wt% Pt) catalyst. A domestic MW oven (Sanyo Company, max. 650 W and 2.45 GHz) was modified by installing a condenser and a thermosensor through the holes on the ceiling. The reaction mixture could be stirred by a magnetic stirrer installed at the bottom of the oven. The temperature of solutions was monitored by a fiber sensor. Into a Pyrex glass flask (200 ml) with a condenser, an aqueous solution of 4-chlorophenol with the dispersed Pt/C catalyst was placed and hydrogen gas (1 aim, 20-30 ml min^-1) was introduced and kept flowing through the flask. In order to avoid violent boiling, the reaction mixture was intermittently irradiated for 1 min followed by a cooling time of 1 min under magnetic stirring. The temperature was raised up to 373 K under MW irradiation for 1 min and went down to 363 K during the cooling time (1 min). For solvent-free dechlorination, i.e., without using water as a solvent, pentachlorophenol or 4-chlorophenol was impregnated on the Pt/C catalyst using its methanol solution. Methanol was removed from the resulting dispersion with a rotary vacuum evaporator. ?Figure 1 clearly shows that the dechlorination rate of 4-chlorophenol in water under MW irradiation was larger than that under the conventional heating. Under MW irradiation, 4-chlorophenol was completely dechlorinated to phenol within 40 min in water. The conventional heating experiment was carried out at 368 K for comparison, but the conversion of 4-chlorophenol was only 50% after 40 min of heating. In addition, the Pt/C catalyst showed steady activity under MW irradiation while its activity was gradually decreased under conventional heating. The sustainable catalysis under MW irradiation can be explained as being due to high temperature of the catalyst that may suppress the poisoning of Pt by adsorption of the increasing chloride anion in the system. In fact, the pH of the water system decreased rapidly with increasing the MW irradiation time because of the formation of HCl in the hydrogenation process. To clarify the high temperature effect, MW-assisted hydrogenation was carried out without using water as a solvent. MW irradiation for 3 min gave 52% conversion of 4-chlorophenol, yielding mainly phenol and cyclohexanol. Small amounts of other unidentified highly hydrogenated chemicals containing no chlorine atoms were also produced. The solvent-free MW irradiation for a longer time (10 min) resulted in perfect conversion of 4-chlorophenol to cyclohexanol and other reduced chemicals. Careful GC-Mass analysis revealed that these chemicals are chlorine-free ones. In the case of pentachlorophenol, 20-min irradiation led to reductive dechlorination at 100% conversion with formation of cyclohexanol and other highly hydrogenated chemicals containing no chlorine atoms. The high efficiency in the reductive dechlorination in the solvent-free systems is attributed to higher temperature of the catalyst surface. Benzene chloride derivatives are of higher reduction potentials than 4-chlorophenol, but they were effectively dechlorinated to benzene at the last step via less chlorinated benzene intermediates. At the same time, MW heating showed its benefit to the catalytic reaction again. The dechlorination rates under MW irradiation were more than two times that under conventional heating. From the same product composition in dechlorination of chlorinated benzenes with or without MW irradiation, it is distinct that MW irradiation only enhances the reaction rate owing to its rapid heating function via activated carbon, and does not give rise to other effects to the thermodynamical equilibrium when the reaction is carried out in liquid state under the present conditions

[en] Full text: The processing of nanosized silver particles can be briefly divided into several regimes: (1) the classical Turkevich preparation of metal colloids, (2) the reversed micelle processes, (3) photoreduction, (4) ultrasonic radiation, and (5) ⁶⁰Co g-irradiation. In the above-mentioned processes, nanosized metal silver particles are synthesized with different morphologies (nanoparticulate, nanowire, and nanoprism) and sizes but only in low concentrations of silver colloids (a few millimoles per liter or less) and in the presence of surfactants as stabilizers. Microwave (MW) dielectric heating is fast emerging as a widely accepted new processing technology for variety of inorganic syntheses due to its penetration and rapid heating. In the MW-assisted syntheses of nanosized nickel and CdS particles, we found that MW heating can effectively control the size distribution of the nanosized nickel and CdS in a narrower range than the conventional heating by thermal convection, and that the effects of MW heating on the morphological structures of the nickel and CdS nanocrystallites are remarkable and attractive. Therefore, we used MW irradiation as a heating source to take advantages of the rapid, selective, and homogeneous, i.e., molecular level, heating in the reaction system for rapid and size-controlled preparation of nanosized metal silver in high concentrations. The as-prepared metal silver was indexed as cubic Ag (JCPDS card, No. 4 - 783) under MW irradiation or by conventional heating. SEM images of samples Al - A6 show that nanosized metal silver particles were formed. Figure 1 graphically expresses the detailed particle size distributions of samples Al - A6. The data of the particle size distributions and average particle sizes of samples A1 - A6 are also listed. When trisodium citrate (1.5 M) and silver nitrate (0.002 or 0.1 M) (A1 - A3) were mixed together, precipitated silver citrate (Ag₃C₆H₅O₇) was immediately formed, which was identified by powder XRD analysis (JCPDS card No. 1-30). The precipitated silver citrate was dissolved after stirring for ca. 10 min, and the reaction solution became clear and colorless even after the addition of formaldehyde (1.0 M), suggesting that citrate anions at a high concentration stabilized silver cations to form solvated complexes. The yields of silver nanocrystallites in the preparation of samples Al and A3 were 53% and 74%, respectively. On the other hand, only trace amount of metal silver was formed after MW irradiation of a silver nitrate -trisodium citrate (A7) or a silver nitrate - formaldehyde (A8) reaction solution for 1 min, respectively, showing the necessity of the coexistence of both sodium citrate and formaldehyde for the reduction of silver cations to form metal silver colloids. MW heating (A1) resulted in the formation of silver nanoparticles with a narrow size distribution and a quite large average particle size when compared to conventional heating (A3) with the same reactant composition. In general, crystal size and size distribution are determined by the both processes of nucleation and crystal growth, which are greatly affected by reaction temperature. MW irradiation can cause a homogeneous (i.e., molecular level) temperature distribution in the reaction solution due to its penetration characteristics, giving uniform nucleation and rapid crystal growth to form narrow size distributed crystallites. It can be concluded that MW irradiation results in the formation of a quite plenty of nuclei by homogeneous and rapid heating of the silver citrate colloids via reduction with formaldehyde to produce small-sized silver crystallites in the successive process of crystal growth by epitaxy

[en] Full text: Nanosized titanium dioxide (TiO₂) is an important industrial material used as a photocatalyst, an efficient solar cell device, and a photonic crystal. As a photocatalyst, the nanosized anatase TiO₂ powder, with a high degree of crystallinity and a large surface area due to a small crystallite size, is regarded important for the enhanced photocatalytic activity. The polyol method was developed over the past two decades and has been applied to the preparation of submicrometer nanoparticles such as metal, metal alloy, metal oxide, binary metal oxide, metal sulfide, metal chalcogenide, and metal telluride in high boiling point polyol media (e.g., ethylene glycol, diethylene glycol). By using the polyol method, Feldmann et al. prepared nanosized TiO₂ particle at 453 K for 2 h in diethlylene glycol. In recent years, the microwave (MW) technology has been successfully applied to a wide variety of chemical reactions and has consequently been the subject of a number of reviews. Recently, the MW technique has been applied to the polyol method, including the preparation of binary oxide nanoparticles in ethylene glycol with KOH at pH 11. In this paper, we show the preparation anatase TiO₂ nanocrystallites in the short time in 1,4-butanediol and 1,5-pentanediol by MW irradiation under the ambient pressure without pH control. This method enables us to also control the crystallite size by varying the co-present amount of water. MW irradiation experiments were performed using a MMG-213VP microwave apparatus (Micro Denshi), equipped with a magnetron (a frequency 2.45GHz and a maximum output power 1.3 kW), an isolator, a power monitor, a three stub tuner, a wave guide and a microwave cavity. The reaction temperature was measured by a thermocouple and was controlled by the intermittent MW irradiation at the constant output power. The irradiation times and the intervals were controlled depending on the deviation of the temperature from the set one. Intrinsically absorbed MW energy was monitored by the forward and reverse power monitor. A water-cooled condenser outside the microwave cavity was connected to a three-neck quartz flask inside the cavity by a quartz joint. A Teflon magnetic stirrer was set in the quartz flask and was driven by a motor. Using 1,4-butanediol or 1,5-pentanediol as a solvent, the product obtained under MW irradiation found the anatase TiO₂ nanocrystallites which was determined by the powder X-ray diffraction (XRD) pattern. The high-resolution transmission electron microscopy (TEM) image showed that the anatase TiO₂ nanocrystallites were aggregated and the 0.35 nm spacing of fringes corresponded to the d value 0.35200 nm of the (101) plane for the anatase TiO₂ (powder diffraction file, PDF ≡21-1272). The size distribution determined for 200 particles selected at random by the dark-field TEM image was centered at 6 nm and ranged between 3 and 10 nm. The average diameter of 5.8 nm was in good agreement with the crystallite size (4.5 nm) calculated from the (101) diffraction peak of the XRD pattern using the Debye-Scherrer equation. The crystallite size prepared in 1,4-butanediol (5.3 nm) was smaller than that in 1,5-pentanediol (8.3 nm) under comparable conditions. With the progressing time from 10 min to 30 min, the crystallite size increased from 7.2 nm to 8.3 nm. The rate of hydrolysis of ≡Ti-O(CH₂)_nOH species (n=4,5), which will be formed by the ligand exchange reaction of ≡ Ti-OiPr and HO(CH₂)_nOH, will be faster in 1,4-butanediol than in 1,5-butanediol. A faster hydrolysis of ≡ Ti-O(CH₂)_nOH species should produce more ≡ Ti-OH species in a shorter period, should cause the homogeneous formation of a large number of seed nuclei and generate smaller TiO₂ nanocrystallites. At setting temperature 453 K, it took 90 min for the sample to show the comparable XRD pattern equivalent to that obtained 30min-reaction at 513 K. The reaction rates of the hydrolysis will be slow at low temperature. When the amount of added water was increased from 1 ml to 3 ml, the crystallite size increased from 4.5 nm to 6.5 nm in 1,4-butanediol, and also increased from 5.9 nm to 10.0 nm in 1,5-pentanediol at 513 K for 30 min under intermittent MW irradiation. The increase of the water concentration will accelerate the polycondensation reactions, thus generate larger TiO₂ nanocrystallites. Therefore, the crystallite size can be controlled by changing the amount of added water. We used 1,4-butanediol and 1,5-pentanediol as solvents, and succeeded in preparing size-controlled TiO₂ nanocrystallites by hydrolysis and polycondensation reactions of Ti(OiPr)₄ by the intermittent MW irradiation under the ambient pressure without pH control. The crystallite sizes ranged from 4.5 nm to 10.0 nm by varying the amount of water. It was easy to control the high reaction temperature by using MW irradiation

[en] The neutron capture cross section of ⁸⁹Y was measured by the time-of-flight method in an energy range from 15 to 100 keV. A pulse-height weighting technique was applied to derive the capture yield. The absolute cross section was determined based on the standard reaction ¹⁹⁷Au(n, γ)¹⁹⁸Au reaction. The neutron capture γ-ray spectrum was derived by unfolding the pulse-height spectrum with detector response functions. (authors)

[en] Single crystals of new alkali niobium and tantalum fluorides, KNaMF₇ (M: Nb, Ta), were prepared by hydrothermal reactions using a perovskite-type oxides, KMO₃ as one of the starting compounds. Their crystal structures were determined from single crystal X-ray diffraction data. These fluorides crystallized in the orthorhombic space group P2₁2₁2₁ (#19) with a = 5.509(2), b = 9.100(4), and c = 11.234(5) Å for KNaNbF₇ and a = 5.517(2), b = 9.093(3), and c = 11.273(4) Å for KNaTaF₇. The crystal structures contained isolated MF₇ polyhedra showing a monocapped trigonal prism geometry and the M⁵⁺ sites could be replaced partially by Mn⁴⁺ ions during the hydrothermal reaction. These Mn⁴⁺ ion-doped fluorides exhibited photoluminescence properties similar to those of red phosphors. The local environment of Mn⁴⁺ ions in these fluorides was confirmed by the X-ray absorption fine structure (XAFS) analysis.