Tsuyoshi Arakawa

Physicochemical properties of rare earth perovskite oxides used as gas sensor material

The change of the conductivity in the rare earth perovskite oxide took place after the chemisorption of flammable gases. The sensitivity for methanol was highest. From the conductivity change of these perovskite oxides after the injection of methanol, the energy needed to promote an electron from a conducting to a nonconducting state, ΔE=Ec-Et, could be derived from the equationσ=Aσ0 exp (−ΔE/kT). LnCoO3 had the smallest ΔE and ΔH (metal-O), which is the binding energy of oxygen coordinating to the metal ions, but exhibited the highest activity for gas sensing. The gas sensing mechanism was also considered.

The catalytic activity of rare earth manganites

Abstract The sequence of catalytic activities for the rare-earth manganites (LnMnO 3 , Ln=La/1 b Eu), La > Pr ⋍ Nd > Sm ⋍ Eu , is correlated with the paramagnetic Weiss constant; it was positive in LaMnO 3 , negative in EuMnO 3 .

Catalytic activity of rare-earth orthoferrites and orthochromites

Abstract The catalytic activity of LnFeO 3 and LnCrO 3 (Ln=Laue5f8Gd) for methanol oxidation has been studied by the measurement of conductivity change on methanol adsorption in the mixture gas ( O 2 (0.5%) + N 2 (99.5%)); The activity of these compounds appeared above the Neel points. The sequence of the activity for LnFeO 3 was Gd > Eu > Sm > Nd > Pr > La, which was correlated with the Neel temperature; GdFeO 3 , which has the lowest Neel temperature, showed the highest activity. However, a relationship between activity and Neel temperature was not clearly observed in LnCrO 3 .

CO gas sensitivities of reduced perovskite oxide LaCoO3 − x☆

Abstract The detection of CO gas was studied using LaCoO 3 − x as a sensing material, which was obtained by reduction of LaCoO 3 in a hydrogen atmosphere at 500 °C. The sensitivity—temperature profile shows a maximum at about 100 °C for an injection of 0.6 ml of CO. CO adsorption caused a decrease in resistivity of LaCoO 3 − x , as in the case of O 2 adsorption. Moreover, the adsorption of CO on LaCoO 3 − x was examined by the temperature programmed desorption (TPD) method.

Catalytic properties and activity of rare-earth orthoferrites in oxidation of methanol

The catalytic properties and activity of LnFeO3 (Ln = La-Gd) in the reaction of methanol oxidation have been studied. LnFeO3 are antiferromagnetic compounds and except for PrFeO3 are an n-type semiconductor. The activity was in the following order: Gd > Eu > Sm > Nd > Pr > La, where the activity was measured at a temperature at which a conversion of methanol to CO2 and H2O became 10%. The relative magnitude of covalency for Feue5f8O bond in LnFeO3 is determined with the measurement of the binding energy of Fe 2p32 in X-ray photoelectron spectra at 380 °C and decreased as the radius of the rare-earth ion decreases. Further, from the measurement of reaction kinetics and conductivity, the mechanism was proposed.

Luminescence properties of Eu ion-exchanged Y-type zeolite

Abstract The luminescent properties of Eu ion incorporated in a Y-type zeolite have been studied. The emission spectrum at room temperature is due to the transition between 5D and 7F levels of the Eu3+ ion. However the emission band for Eu3+ decreased as this compound was degassed at above 300°C and the band emission, peaked at 445 nm, appeared. This band emission is associated with Eu2+ 5d to 4f transition.

The direct reaction between nitric oxide and the superconductor YBa2Cu3O7−δ

The interesting properties of YBa/sub 2/Cu/sub 3/O/sub 6.83/(NO)/sub chi/ obtained by the direct reaction with nitrogen monoxide (NO) gas are described. A large amount of NO was rapidly absorbed by the YBa/sub 2/Cu/sub 3/O/sub 6.83/ superconductor at around 573 {Kappa}. The maximum NO content so far achieved corresponded to chi {approx equal} 2.5. Up to chi {approx equal} 1.0 the absorption rate was fast, although in order to obtain a composition of chi {approx equal} 2.5 several days were required. The implanted samples were not superconducting, for example the resistivity of a compound (chi=0.5) increased with decreasing temperature. No incorporation was found to shift the crystal symmetry from orthorhombic to tetragonal. At chi = 2.5, a tetragonal phase decomposed to form various solid products such as CuO,Y/sub 2/BaCuO/sub 5/, BaCuO/sub 2/, Ba(NO/sub 2/)/sub 2/, etc.

Reduction of perovskite oxide LnCoO3(Ln = La-Eu) in a hydrogen atmosphere

The reduction of LnCoO3(Ln = La-Eu) in a hydrogen atmosphere of 2×106 Pa at 25 to 600° C was investigated by X-ray diffraction and thermogravimetric analysis, which were carried out to pursuein situ the reaction of reduction of LnCoO3. The amount of lattice oxygen consumed at 600° C increases on going from LaCoO3 to EuCoO3. It was shown that the reduction process proceeded through the formation of a series of oxygen-deficient structures in keeping the perovskite structures; for example, the final X-ray powder diffraction pattern of NdCoO3−x (x=1.1) could be indexed on a cubic cell (a = 0.39 nm) and an orthorhombic cell (a0=0.522 nm,b0 = 0.559nm, c0 = 0.795nm), the relationship with the cubic cell beinga0 ≃ 21/2ac,b0 ≃ 21/2ac,c ≃ 2ac.

Surface study of a Ag electrode on a solid electrolyte used as oxygen sensor

Abstract The Ag electrode surface of a solid electrolyte has been studies by X-ray photoelectron spectroscopy and a scanning electron microscope. The XPS has performed in situ observation of the Ag electrode surface when DC current was passed through a system of P O 2 ,Ag∣ZrO 2 -CaO∣Ag, P O′ 2 . The nature of the Ag-O system on the electrode is dependent on the temperature. It has been observed by XPS that there are three types of Ag-O system at 400°C. Type I with O ls binding energy of 532.6 eV only appeared at 325°C. Moreover, when the temperature was raised to 400°C, Types II and III (with BEs 531.1 and 529.2 eV) appeared. And a fourth state at ca. 530.1 eV also existed. Type I is the type where the mixing of the 2p orbitals with the d-band of the Ag metal is not so strong. Type II is assigned as a diatomic oxygen-silver complex, i.e. Ag + -O - 2 . Type III is considered to represent oxygen atoms in an oxide-like layer. However, the O ls signal disappeared switching off the DC current in both cases. A scanning electron microscope study of the electrode texture helped to understand the change of O ls signal.

Redox behaviour of europium Ion-exchanged zeolites

In Euue5f8Y and Euue5f8M zeolites degassed above 300°C, a part of Eu3+ ion is reduced to Eu2+ ion. The order of which Eu3+ ion in zeolites is susceptible to reduction under evacuation at 600°C is Euue5f8M > Euue5f8Y > Euue5f813X ⋍ Euue5f83A. The emission spectra in Euue5f8Y and Euue5f8M showed that the Eu2+ species increase upon degassing and they decrease after the adsorption of degassed water. In the adsorption of water to an activated Euue5f8Y and Euue5f8M zeolite, the evolved gas was identified as hydrogen. A mechanism is proposed that accounts for the observations.