Hideo Imoto

The Active Phase of Nickel/Ordered Ce2Zr2OxCatalysts with a Discontinuity (x=7-8) in Methane Steam Reforming

In recent years, there has been a surge in interest in syngas (H2/CO) and H2 production technologies, which utilize a wide variety of hydrocarbon feed stocks, such as gasoline, diesel, LPG, natural gas, methanol, and bio-ethanol. Among fossil fuels, natural gas ( 90 vol% CH4) is the ideal fuel, owing to its ready availability, high energy density, and wide distribution network; CH4 activation and reforming provide attractive ways to produce syngas, which can be transformed to useful larger hydrocarbons. Catalysts based on both noble metals and other metals have been extensively studied for CH4 steam reforming. [1, 2] Noble-metal (Rh, Ru, Ir, Pd, and Pt) catalysts are active and stable; however, because of the limited supply and high cost of noble metals, much attention has been paid to the development of non-noble metal catalysts, among which nickel-based catalysts have attracted particular attention because of their similar mechanistic features to noble-metal catalysts. The strong C H bonds of CH4 (439 kJmol ) and endothermic heat of reforming reactions necessitate high temperatures for practical CH4 conversion, and thus stable catalysts that resist sintering under extreme operating conditions. In CH4 steam reforming, coke formation that deactivates the catalyst is thermodynamically favored at a H2O/CH4 ratio less than 1.4. Thus, industrial CH4 steam reforming is usually carried out at a H2O/CH4 ratio of 1.4 or greater. Although catalytic CH4 steam reforming at low H2O/CH4 ratios have many advantages from operational and energy-consuming viewpoints, conventional nickel-based catalysts suffer from severe carbon deposition under such conditions. Supports and additives (for example, CeO2, ZrO2, CeO2-ZrO2, and La2O3) have been used to confer catalysts with kinetic resistance to carbon deposition and Ni sintering because they enhance redox activity and thermal stability, thereby promoting steam reforming. The efficient CH4 upgrading has long been a challenge in fundamental research. Herein, we report the unique properties and active phase of a new Ni/ordered Ce2Zr2Ox (x = 7–8) catalyst with a regular arrangement of Ce and Zr ions in CH4 steam reforming to produce H2 and CO at H2O/CH4 = 1. The catalytic performance of Ni/Ce2Zr2Ox (x = 7–8) strongly depends on the phase and oxygen content of Ce2Zr2Ox, and it shows a unique discontinuity in catalytic activity at x = 7.5. The 2 wt % Ni/pyrochlore-Ce2Zr2O7 catalyst showed a remarkable performance in CH4 steam reforming at 923 K at H2O/CH4 = 1 (Table 1). Ni/CeO2, Ni/ZrO2, and Ni/CeO2ZrO2 reduced by H2 were much less active and selective than Ni/Ce2Zr2O7, and significant deactivation was observed probably owing to Ni sintering and carbon deposition. On the other hand, the Ni/pyrochlore-Ce2Zr2O7 catalyst was stable, resulting in a remarkably high catalytic performance (for a typical 50 h performance, see the Supporting Information, Figure S4). At 973 K, the Ni/Ce2Zr2O7 catalyst exhibited high CO selectivity of 96–98% and high H2 selectivity of 96– 99% at CH4 and H2O conversions of 92–94% and > 96 %, respectively. Platinum, a typical noble metal active for CH4 steam reforming, was supported on CeO2, ZrO2, or Ce2Zr2O7, but the performance was not significantly enhanced by these types of supports, and the CH4 conversion on these Pt-based catalysts ranged between 29 % and 39 %. For Pt-based catalysts, CH4 steam reforming may be controlled by Pt rather than by the nature of the support. 8] In the presence of 0.8% O2 in the reaction feed, the Ni/ Ce2Zr2O7 catalyst also exhibited high H2 selectivity (97–99%) at a CH4 conversion of 93–94 % for at least 10 h. No significant deactivation was observed. These are great advan[*] Dr. M. Tada, Dr. S. Zhang, N. Ishiguro Institute for Molecular Science 38 Nishigo-naka, Myodaiji, Okazaki, Aichi 444-8585 (Japan) E-mail: mtada@ims.ac.jp

Charge Compensation Mechanism Decreases Dielectric Loss in Manganese-Doped Pb(Fe1/2Nb1/2)O3 Ceramics

Mn-doped Pb(Fe1/2Nb1/2)O3 (PFN) ceramics are synthesized by the conventional solid-state reaction method via a B-site oxide mixing route. The pure perovskite phase of Mn-doped PFN is obtained up to 0.75 mol% of manganese doping. Due to manganese doping, dielectric loss of the obtained ceramics decreases continuously up to 3 mol% of manganese, although pyrochlore phase appears. The decrease of dielectric loss can be explained by the charge compensation effect caused by the substitution of Mn4+ for Fe3+ in the octahedral cage of the perovskite structure, which is confirmed by X-ray photoelectron spectrometry (XPS). That is, due to the substitution of Mn4+ for Fe3+ ions, the relative content of iron in different valence states changes, i.e., the relative content of Fe2+ ions increases with the amount of doped manganese. Therefore, the hole-conduction mechanism caused by the partial reduction of Fe3+ to Fe2+ in the PFN system weakens, which leads to the decrease of dielectric loss. The effect of manganese doping on the depression of mechanical vibration loss also provides some contribution to the decrease of total dielectric loss through pinning down the rotation of spontaneous polarization induced by combining Mn2+ (reduced of doped Mn4+ ions during sintering) with the oxygen vacancy (produced by the compensation effect) and standing near the domain boundary. The relative dielectric constant of the Mn-doped PFN ceramics also decreases continuously with the increase of manganese content, which is believed to be related to the reduction of space-charge polarization.

Transparent conductive Cd3TeO6 thin films with perovskite structure

Transparent conductive Cd3TeO6 thin films were deposited by radio-frequency magnetron sputtering using a target composed of CdO and TeO2 powders, and these films’ electrical and optical properties were examined. The electrical resistivity of 1.9×10−2Ωcm and an average transmittance above 85% in the visible region (400–800 nm) were obtained after annealing the film at 500 °C. The film’s carrier density and Hall mobility were 8.7×1019cm−3 and 6.8cm2V−1s−1, respectively. The absorption edge of the films was shifted to a lower wavelength by increasing the carrier density. The maximum band gap of these films was found to be 3.69 eV.

Ferroelectric Phase Transition in Perovskite Oxide CdTiO 3

Single crystals of CdTiO 3 were synthesized by a flux method. The obtained single crystals showed the orthorhombic perovskite structure with the space group Pnma at room temperature, and a dielectric anomaly appeared near 80 K, which was similar to the polycrystalline samples. The temperature dependence of the dielectric constants along three directions was investigated, and a dielectric anisotropy was observed. The comparison of the measured dielectric constants indicates that the polarization of CdTiO 3 is directed along the b axis of the space group Pnma .

New n-Type Thermoelectric Oxide, Cd3TeO6

Electron doping into Cd3TeO6 with a 1:1 ordered perovskite-type structure was carried out by two methods: the introduction of oxygen vacancies, and the atomic substitution of In for Cd. Both methods turned polycrystalline samples into metallic conductors while single crystals with metallic conductivity were obtained by the second method. The resistivity and thermoelectric power of the single-crystal indium-substituted sample were 0.6 mΩcm and -50 µVK-1, respectively, giving a power factor of 4×10-4 WK-2m-1, which is the highest among nontransition metal oxides.

Reduction of Dielectric Losses in Pb(Fe1/2Nb1/2)O3-Based Ferroelectric Ceramics

Pseudoternary Pb(Fe1/2Nb1/2)O3–PbZrO3–PbTiO3 (PFN–PZ–PT) ferroelectric ceramics with a pure perovskite phase were fabricated by a conventional solid-state reaction method via a B-site oxide mixing route in the whole composition range. The ceramics obtained exhibit marked frequency-dependent dielectric behavior and a large dielectric loss when the concentration of Pb(Fe1/2Nb1/2)O3 exceeds 40 mol %. The large dielectric loss is thought to correlate with the thermally activated hole conduction induced by the partial reduction of Fe3+ to Fe2+ ions during sintering, which can be greatly decreased by adding a small amount of MnO2 or by cycling under an alternating electric field. The values of the dielectric constant and the frequency-dependent dielectric behavior at temperatures around and below the temperatures of the dielectric maximum (Tm) are reduced accordingly. The decrease in dielectric loss is believed to be related to the ionization of the manganese-substituted donor defects. The effect of manganese doping on the depression of mechanical vibration loss also contributes little to the decrease in total dielectric loss through pinning down the rotation of space-charge polarization induced by the combining of complex manganese-substituted defects (Mn2+) with oxygen vacancies and standing near the grain boundaries.

Improvement of Dielectric Properties in Iron-Containing Ferroelectrics

Iron-containing ferroelectrics tend to exhibit large dielectric loss accompanied by strong frequency dispersion of the dielectric properties, which is considered to correlate with the partial reduction of Fe3+ to Fe2+ ions during sintering. After sintering in an O2 atmosphere, MnO2 and Li2CO3 dopings exhibit superior behavior in decreasing the dielectric loss of the iron-containing dielectrics. The strong frequency dispersion of the dielectric response and the abnormally recurrent increase of the dielectric constant and loss in the paraelectric region are correspondingly suppressed. An electron compensation mechanism is advanced to interpret the reason for the improvement in dielectric properties in the iron-containing dielectrics, in which the generation of donor electrons is based on different ionization paths.

Ferroelectric Phase Transition in CdTiO 3 Single Crystal

Single-crystal X-ray diffraction measurements at low temperature were carried out using the image-plate technique in the vacuum chamber at BL02B1 of SPring-8. The intensity data were collected at 20 K, 70 K, and 150 K, 36 photographs being taken at each temperature. The systematic extinctions observed at 20 K could be summarized as h = 2 n + 1 for ( hk 0) reflections. This observation together with the observations of the ferroelectricity at this temperature uniquely determine the space group as P 2 1 ma though a previous report suggested the space group to be Pn 2 1 a . The structure could be refined with this space group yielding R = 0.056 for 4359 reflections. The structure of CdTiO 3 at 150 K was found to be almost identical to the previously reported one, belonging to the space group Pnma .

Powder X-ray diffraction study of ferroelectric phase transition in perovskite oxide CdTiO3

Abstract The ferroelectricity of perovskite oxide CdTiO3 was ascertained by the measurements of the dielectric constant, D-E loops, and pyroelectricity. The ferroelectric Curie temperature TC was about 80 K and the residual electrical polarization was 0.2 μC/cm2. Below TC , the temperature dependence of the lattice parameter a turned negative while the parameters b and c showed very small temperature dependence. The Rietveld analyses of the powder X-ray diffraction date at 15 K were carried out for the space groups Pnm21, Pn21 a and P21 ma, which are noncentrosymmetric subgroups of the space group of the paraelectric structure. The spontaneous polarizations calculated from the refined structural parameters indicated that the space groups Pn21 a and P21 ma were possible for the ferroelectric state of CdTiO3.

Crystal Structures and Properties of Europium Aluminum Oxynitride Eu2AlO3.75N0.1 and Europium Aluminum Oxide EuAl2O4

The reactions among Eu2O3, AlN, and Al2O3 with the ratios Eu:Al = 2:1 and 1:2 at 1200 °C for 10 h yielded Eu2AlO(3.75)N(0.1) and EuAl2O4, respectively. The powder X-ray diffraction pattern of the new oxynitride could be indexed as a monoclinic structure with the space group I2 (No. 5) (a = 3.7113(2) Å, b = 3.6894(2) Å, c = 12.3900(8) Å, and β = 90.6860(5)°). This structure was found to be a novel distorted Ruddlesden-Popper type. For EuAl2O4, isostructural with monoclinic SrAl2O4 (space group P2(1), No. 4), a structural refinement was performed, indicating that the cell parameters were a = 8.44478(11) Å, b = 8.82388(12) Å, c = 5.15643(7) Å, and β = 93.1854(12)°. (151)Eu Mössbauer spectra revealed that the divalent and trivalent Eu ions coexisted in Eu2AlO(3.75)N(0.1), while Eu ions were in the divalent state in EuAl2O4. A photoluminescent mechanism due to 4f(7) ((8)S(7/2)) ↔ 4f(6)5d(1) of europium in EuAl2O4 was proposed on the basis of the calculated band structure, the band gap obtained from UV-vis diffuse reflectance spectra, and the photoluminescence spectra.