Yasutake Teraoka

Mixed ionic-electronic conductivity of La1−xSrxCo1−yFeyO3−δ perovskite-type oxides

Ionic (σi) and electronic (σe) conductivities of mixed conductive La1−xSrxCo1−yFeyO3−δ were separately measured by means of fourprobe ionic dc and ordinary four-probe dc techniques, respectively. At 1073 K, for instance, σi ranged in the order of 1 – 10−2 S cm−1 while σe was around 102 S cm−1, indicating that the present oxides were good mixed conductors with ionic transport number 10−2 – 10−4. σi increased as contents of Sr and Co increased though Sr content was more influential. The apparent activation energy for oxide ion conduction as well as the dependences of σi on oxide composition and oxygen partial pressure suggested that oxide ion conduction occurred via a vacancy mechanism. The relation of mixed conductivity to the oxygen semipermeability was also discussed.

Oxidation catalysis of perovskites --- relationships to bulk structure and composition (valency, defect, etc.)

In this study, oxidation catalysis of perovskites, which is the most important catalytic application of the oxides, will be described with particular emphasis on its relation with solid-state chemistry

Electronic interaction between metal additives and tin dioxide in tin dioxide-based gas sensors

The electronic interaction of SnO2 with Ag and Pd particles dispersed on its surface was examined by means of X-ray photoelectron spectroscopy (XPS). The binding energies (BE) of Sn3d and O1s of Ag(1.5 wt%)-SnO2 and Pd(3.0 wt%)-SnO2, which were lower by 0.5–0.7 eV than those of pure SnO2 in the as-prepared state, shifted reversibly by reduction and oxidation treatments. These shifts in BE are shown to reflect the shifts of Fermi energy of SnO2 which are interacting electronically with the metal additives. The electronic interaction depended on the metal loading, being strongest at 1.5 wt% and 3.0 wt% loadings of Ag and Pb, respectively. The implication is that the electronic interaction is of primary importance to the inflammable gas detection by Ag-SnO2 sensors.

Oxygen sorption and catalytic properties of La1−xSrxCo1−yFeyO3 Perovskite-type oxides

Abstract Oxygen desorption from La 1− x Sr x Co 1− y Fe y O 3 perovskite-type oxides below 850°C was examined by a temperature-programmed desorption (TPD) technique. The catalytic activities of the oxides for the combustion of n -butane and methane as well as H 2 O 2 decomposition in an alkaline solution were also measured and were discussed in relation to oxygen sorption properties. The amount of oxygen desorbed increased with increasing Sr content at a fixed B-site composition, for which an increase in the number of oxide ion vacancies with x was responsible. On the other hand, the total amount of oxygen desorbed was hardly affected by B-site composition, but partial substitution of Fe for Co enhanced desorption and sorption of oxygen particularly in the low-temperature region. Substitution of Sr for La as well as of Fe for Co was found to promote catalytic activity, though the activity changed with oxide composition in a manner dependent on the kind of reactant. For n -butane combustion over La 1− x Sr x Co 0.4 Fe 0.6 O 3 , maximum activity was obtained at x = 0.2. La 0.2 Sr 0.8 Co 1− y Fe y O 3 activity was highest at y = 0.4 for n -butane combustion, while it was independent of y for methane combustion. For H 2 O 2 decomposition in an alkaline solution, on the other hand, La 1− x Sr x Co 0.4 Fe 0.6 O 3 and La 1− x Sr x CoO 3 activities increased monotonically with an increase in x , and the former oxides were more active than the latter. These variations in catalytic activities with oxide composition were well accounted for by the oxygen sorption properties revealed by TPD.

H2S Poisoning of Solid Oxide Fuel Cells

The influence of H 2 S fuel impurity on power generation characteristics of solid oxide fuel cells (SOFCs) has been analyzed by measuring cell voltage at a constant current density, as a function of H 2 S concentration, operational temperature, and fuel gas composition. Reversible cell voltage change was observed around 1000°C, while fatal irreversible degradation occurred at a lower operational temperature, at a higher H 2 S concentration, and at a lower fuel H 2 /CO ratio. Sulfur tolerance of SOFCs was improved by using Sc 2 O 3 -doped ZrO 2 instead of Y 2 O 3 -doped ZrC 2 as electrolyte and/or as electrolyte component in the anode cermets. It has been found that H 2 S poisoning consists of at least two stages, i.e., an initial cell voltage drop within a short time period to a metastable cell voltage, followed by a gradual larger cell voltage drop. Possible H 2 S poisoning processes are discussed.

Simultaneous catalytic removal of nitrogen oxides and diesel soot particulate over perovskite-related oxides

Abstract Catalytic performance of perovskite-type (ABO3) and K2NiF4-type (A2BO4) oxides for the reduction of NOx by diesel soot particulate in the presence of excess oxygen, that is, the simultaneous removal of NOx and soot, has been investigated. Their catalytic activity, evaluated from the ignition temperature of soot and selectivity to NOx reduction into nitrogen largely depended on both A-site and B-site cations and the partial substitution of potassium at A-sites prominently promoted both the activity and selectivity. These mixed metal oxide catalysts were superior to transition metal simple oxides and Pt/A12O3 in the selectivity for NOx reduction. The ignition temperature of soot in NO + O2 gas stream was lower than that in NO or O2 gas stream, implying that nitrogen dioxide accelerates the oxidation or activation of soot.

Synthesis of LaKMnO perovskite-type oxides and their catalytic property for simultaneous removal of NOx and diesel soot particulates

Abstract Mixed metal oxides in the LaKMnO system were synthesized and their catalytic properties toward the simultaneous NO x –soot removal reaction were investigated. The solubility limit of K in La 1− x K x MnO 3 was determined between 0.2 and 0.25 from X-ray diffraction (XRD) measurement and crystal structure investigation, and K 2 Mn 4 O 8 was by-produced beyond the solubility limit. The activity and the NO x reduction selectivity depended significantly on the K content, and the oxides with intermediate K contents ( x =0.2 and x =0.25) or with the composition close to the solubility limit were good catalysts with higher activity and selectivity. The present study revealed that the LaKMnO oxides are good candidates of catalysts for the simultaneous NO x –soot removal reaction.

Oxygen sorption and catalytic properties of La sub 1 minus x Sr sub x Co sub 1 minus y Fe sub y O sub 3 perovskite-type oxides

Abstract Oxygen desorption from La 1− x Sr x Co 1− y Fe y O 3 perovskite-type oxides below 850°C was examined by a temperature-programmed desorption (TPD) technique. The catalytic activities of the oxides for the combustion of n -butane and methane as well as H 2 O 2 decomposition in an alkaline solution were also measured and were discussed in relation to oxygen sorption properties. The amount of oxygen desorbed increased with increasing Sr content at a fixed B-site composition, for which an increase in the number of oxide ion vacancies with x was responsible. On the other hand, the total amount of oxygen desorbed was hardly affected by B-site composition, but partial substitution of Fe for Co enhanced desorption and sorption of oxygen particularly in the low-temperature region. Substitution of Sr for La as well as of Fe for Co was found to promote catalytic activity, though the activity changed with oxide composition in a manner dependent on the kind of reactant. For n -butane combustion over La 1− x Sr x Co 0.4 Fe 0.6 O 3 , maximum activity was obtained at x = 0.2. La 0.2 Sr 0.8 Co 1− y Fe y O 3 activity was highest at y = 0.4 for n -butane combustion, while it was independent of y for methane combustion. For H 2 O 2 decomposition in an alkaline solution, on the other hand, La 1− x Sr x Co 0.4 Fe 0.6 O 3 and La 1− x Sr x CoO 3 activities increased monotonically with an increase in x , and the former oxides were more active than the latter. These variations in catalytic activities with oxide composition were well accounted for by the oxygen sorption properties revealed by TPD.

Equilibria in Fuel Cell Gases I. Equilibrium Compositions and Reforming Conditions

Using thermochemical data of ca. 300 compounds consisting of carbon, hydrogen, and oxygen, the amounts of equilibrium products have been calculated for various fuel cell fuels including alkanes (CH 4 , C 3 H 8 , C 8 H 1 8 , C 1 2 H 2 6 ), alcohols (CH 3 OH, C 2 H 5 OH, C 3 H 7 OH), alkenes, alicyclic hydrocarbons, and dimethyl ether, as well as for other hydrocarbon-containing fuels such as biogas and coke oven gas, in the temperature range between 100 and 1000°C. It has been resealed that the major constituents in typical fuel cell gases in thermodynamic equilibrium are H 2 (g), H 2 O(g), CO(g), CO 2 (g). CH 4 (g), and C(s). Possible minor constituents are specified while their concentrations were negligibly low. Derived are the minimum amounts of H 2 O and CO 2 for reforming and O 2 for partial oxidation, thermodynamically essential to prevent carbon deposition.

Kinetics of soot—O2, soot—NO and soot—O2—NO reactions over spinel-type CuFe2O4 catalyst

Abstract Kinetics of soot-O 2 , soot-NO and soot-O 2 -NO reactions over CuFe 2 O 4 were investigated by using temperature programmed reaction (TPR) technique in which a soot-catalyst mixture was loaded in a reactor and exposed to a fluent stream containing gaseous reactant(s) under a constant heating rate. Within the temperature range where a substantial amount of the charged soot remained and the reaction can be regarded to be the zero order in the amount of soot, the kinetic analysis of non-steady TPR results was possible. The half-order kinetics in the partial pressure of O 2 ( P O 2 ) was obtained for the soot-O 2 reaction. The soot-NO reaction was complicated in the temperature dependence of CO 2 and N 2 formation as well as in the reaction order with respect to P NO ; the reaction order was first at lower temperatures and increased with increasing temperature. In the soot-O 2 -NO reaction, i.e. the so-called simultaneous soot-NO x removal, the rate of CO 2 formation and those of both N 2 and N 2 O formation depended on partial pressures as P 0.6 NO P 0.6 O 2 and P 1.0 NO P 0.4 O 2 , respectively. Based on the kinetic results, possible reaction mechanisms were discussed.