研究報告　-　31巻　(2011年)

総説 - Review

31-1

密度汎関数理論(DFT)に基づいたRu/Tio₂ 触媒の反応解析に関する理論研究
〜CO₂ベンド吸着構造〜
Theoretical study on reaction analysis of Ru/TiO₂ catalyst based on density functional theory (DFT): CO₂ molecule in a bent form

島崎智実、赤丸悟士、阿部孝之、久保百司
T. Shimazaki, S. Akamaru, T. Abe, M. Kubo

論文 - Original

31-2

多角バレルスパッタリング法により調製したPd/ZnO触媒の メタノール水蒸気改質反応特性
Methanol Steam Reforming Using Pd/ZnO Catalyst Prepared by the Polygonal Barrel-Sputtering Method

團野雅斗、松原圭佑、井上光浩、阿部孝之
M. Danno, K. Matsubara, M. Inoue, T. Abe

31-3

多角バレルプラズマ表面改質法による金属チタン微粒子の表面窒化
Surface Nitridation of Titanium Particles by Polygonal-Barrel Plasma Treatment System

松原圭佑、團野雅斗、井上光浩、阿部孝之
K. Matsubara, M. Danno, M. Inoue, T. Abe

31-4

Zr(Mn_0.5Cr_0.5 )₂の活性化と水素吸収挙動
Activation of Zr(Mn_0.5Cr_0.5)₂ and its hydrogen absorption behavior

原　正憲、荻野慎平、西村克彦、松山政夫
M. Hara, S. Ogino, K. Nishimura, M. Matsuyama

31-5

バナジウム表面における水素の解離反応速度の バルク中酸素濃度依存性
Sticking of H₂ on Oxygen-Covered Surface of Vanadium Doped with Oxygen to Various Bulk Concentrations

波多野雄治、A. Livshits、 A. Busnyuk、V. Alimov、松山政夫
Y. Hatano, A. Livshits, A. Busnyuk, V. Alimov, M. Matsuyama

31-6

高密度・低エネルギー重水素プラズマに曝露したタングステンの 表面トリチウム濃度分布
Tritium concentration in tungsten surface exposed to low-energy, high-flux D plasma

磯部兼嗣、V. Kh Alimov、山西敏彦、鳥養祐二
K. Isobe, V. Kh Alimov, T. Yamanishi, Y. Torikai

技術報告 - Technical Report

31-7

トリチウム曝露時にスペーサーとして用いられたタンタル線の除染
Decontamination of Tantalum Wire Loaded with Samples as Spacer During a Tritium Exposure

阿部信介、松山政夫
S. Abe. M. Matsuyama

ノート - Note

31-8

金属水素化物用交流磁束計の構築とその性能評価
Construction of Alternating-Current Magnetometer for Metal Hydride and Its Performance Tests

赤丸悟士、岡�ｱ圭祐、原　正憲、松山政夫
S. Akamaru, K. Okazaki, M. Hara, M. Matsuyama

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研究報告31 - 1
総説 - Review

The Ru/TiO₂ catalyst holds great promise as a key material for artificial conversion of CO₂ to CH₄ through several hydrogenation elementary reactions. In this study, we examined the bent structure of CO₂ molecules on the Ru(0001) surface and Ru-nano-clusters to investigate the Ru/TiO₂ catalytic reaction, based on the density functional theory (DFT) at the generalized gradient approximation (GGA) level under the periodic boundary condition. The bent structure of CO₂ can trigger a series of hydrogenation reactions leading to the formation of CH₄, and this structure can be observed not only on the Ru (0001) surface and the Ru nano-particles on the TiO₂(101) surface but also on the homogeneous Ru complex catalyst. These findings suggest that the bending of the CO₂ molecule on Ru metal is a key process to activate the stable CO₂ molecule.

A Pd/ZnO sample prepared by the polygonal barrel-sputtering method was tested for the performance in methanol steam reforming in comparison with a similar sample from a conventional wet process. Transmission electron microscopy (TEM) showed that sputtering produced smaller and more uniformly sized Pd nano-particles than the wet process. Steam reforming of methanol was conducted by supplying a gaseous mixture of methanol + H₂O + Ar. In the experiment with the polygonal barrel-sputtering method, H₂ generation started at a lower temperature (80℃) than that for the wet process (100℃). In addition, H₂O was scarcely consumed at both of the samples when H₂ was generated at < 100 ℃. These results suggested that the sputtering preparation allowed direct decomposition of methanol to occur easily at less than 100℃, as supported by the observed relationship between the CO/CO₂ generation ratio and the reaction temperature. It should be noted that the smaller Pd particle sizes for the sputtering method were maintained at reaction temperatures up to 150 ℃.

  We demonstrated surface nitridation of titanium particles using the polygonal barrel-plasma treatment system. In order to verify the formation of TiN, a Ti plate was treated in our plasma treatment system. The color of the Ti plate changed from metallic to gold as a result of N₂ plasma treatment. Glow discharge optical emission spectrometry of the plasma-treated Ti plate confirmed that nitrogen was inserted. From XPS measurements, it was also found that the nitrogen in the sample was ascribed to the formation of Ti-N bonds. It should be noted that the X-ray diffraction patterns of the Ti plate before and after the plasma treatment showed only the reflection peaks of Ti metal, supporting that the Ti-N bond formation occurred only on the surface of the plate. Subsequently, N₂ plasma treatment was applied to Ti particles. The plasma treatment using a swinging motion of the barrel containing Ti particles changed the color of the particles uniformly from gray to brown. The UV-VIS absorbance spectrum of the plasma-treated Ti particles was similar to that of the plasma-treated Ti plate. This result was considered a proof of the Ti-N bond formation on Ti particles achieved by the polygonal-barrel plasma treatment system.

  To obtain the isotherms of Zr(Mn_1-xCr_x)₂ - H system, the activation process was studied. The Zr(Mn_0.5 Cr_0.5 )₂ sample prepared was heated at 873 K for 2 hrs in vacuum as the activation process for isotherm measurements. The isotherm at ambient temperature had a single plateau. However, the width of the plateau region decreased in the course of repetitive isotherm measurements, and the plateau disappeared by the 5th measurement. The disappearance of the plateau region would be explained by the surface poisoning due to the chromium oxide yielded by the activation process.

  The sticking probability of H% on the surface of polycrystalline V covered with oxygen was examined as a function of oxygen concentration in the bulk metal (0.4 to 3.3 at.%) at specimen temperatures from 200 to 1000℃. At all temperatures examined, the sticking probability was strongly dependent on the oxygen concentration in the bulk. Such correlation between the reaction rate of H₂ on the surface and the oxygen concentration in the bulk was attributed to the dependence of concentration of oxygen vacancies (holes in surface oxygen coverage) on the oxygen bulk concentration. The values of sticking probability were relatively high even at the oxygen bulk concentration of 3.3 at.% the oxygen coverage on vanadium was rather “holey” even at the oxygen concentration close to the solubility limit. The activation energy for H2 dissociation on such holes in the coverage was evaluated to be 10 kJ/mol H₂.

  The limits on tritium inventory in the vacuum vessel and the need for prevention of impurity ingress into plasma make plasma-surface interaction on tungsten an important issue. It is well known that plasma exposure on tungsten makes some kinds of blisters on the surface and increases the hydrogen inventory. On the other hands, there is a possibility that plasma exposure would change the characteristic of surface and surface region in tungsten and cause the increase of tritium inventory. To understand the change of tritium inventory in the surface region and surface in tungsten by plasma exposure, tritium concentration in tungsten exposure by low-energy (38eV), high-flux D plasma with was examined with BIXS after thermal exposure of tritium gas. D plasma exposure was carried out at specimen temperature around 495 and 550K. After that, specimens were exposed to gaseous tritium diluted with deuterium at 473K for 3 hours. The tritium concentration was measured with BIXS. The tritium concentration in surface and surface region was found to be increased by plasma exposure. And its concentration of tungsten exposed at 495 K was estimated to be twice higher than that of as-received tungsten.

  A tantalum wire was contaminated with tritium unexpectedly generated during a tritium exposure experiment on ceramic materials. The amount of tritium retention in the tantalum wire was estimated to be 13.3 GBq, and 88% of this amount was recovered as tritium gas by heating at 943 K. After additional evacuation at the same temperature, a part of the wire was dissolved in an aqueous solution of hydrofluoric acid. The result showed that heating was able to decrease the residual amount of tritium in the tantalum wire finally to 564 kBq. Subsequently, a part of the tantalum wire was immersed in pure water for about one month to examine the effect of water decontamination. This removed 37% of the residual tritium in the tantalum wire. Altogether the contamination level decreased to 355 kBq, which was 2.7×10^-3% of the initial tritium amount in the wire.

   An alternating-current magnetometer was assembled to measure the magnetic susceptibility of metal hydride in hydrogen atmosphere, and its performance tests were carried out. The magnetometer was designed to be integrated into the conventional pressure-composition-temperature (PCT) measurement system without any reconstruction. Calibration of the magnetometer was performed using a paramagnetic Gd₂O₃ powder, and it gave a linear relationship between the output signal and the magnetic moment of Gd₂O₃ powder. The magnetic susceptibility of PdH_x (0 ≦ × ≦ 0.7) was measured simultaneously with the PCT curve. The magnetic susceptibility of Pd hydride decreased with increasing hydrogen concentration in Pd and finally reached zero at the hydrogen concentration above 0.65. The behavior of magnetic susceptibility and PCT curves quantitatively agreed with the previously reported values.