C.B. Alcock

Perovskite electrodes for sensors

Abstract The data published on perovskite oxides La1−xSrxBO3−δ (B = Co, Cr, Fe, Mn) are compared to assess their use as electrodes in applications such as fuel cells, low temperature oxygen meters, etc. The oxides are reviewed in terms of their stability in temperature and oxygen partial pressure of operation, nonstoichiometry, conductivity and catalytic properties and the effect of the strontium substitution content, x. From this comparison it is found that La1−xSrxCrO3−δ is better suited for applications in reducing conditions, while La1−xSrxCoO3−δ is better suited for applications at higher oxygen potentials. While noble metals like platinum are currently being used for these applications, the perovskite oxides offer better kinetics at lower temperatures. To compare the kinetics, the electrochemical cell: air, electrode/ Bi-based or Zr-based electrolyte/electrode, air was used and the overpotential of the interfaces was measured. The electrodes used for this comparison are Pt, La0.7Sr0.3MnO3−δ, and La0.7Sr0.3CoO3−δ. The results show that for temperatures lower than 800°C, the oxide electrodes performed better than platinum. This is attributed to the ease of transfer of oxygen potential between the electrodes and the electrolyte.

Surface characterization and catalytic properties of perovskite type solid oxide solutions, La0.8Sr0.2BO3 (B = Cr, Mn, Fe, Co or Y)

The surface compositions of La0.8Sr0.2BO3 (B = Cr, Mn, Fe, Co or Y) were investigated by X-ray photoelectron spectroscopy (XPS). Among the five solid oxide solutions, the yttrium showed the highest concentration in the surface atomic ratio [B-ion(La + Sr)]. Furthermore, the O1s spectra characteristics of yttrium perovskite are found to be different from those of the other solid solutions with the peak shifted to higher binding energy value. The electrical conductivity of La0.8Sr0.2CrO3 and La0.8Sr0.2YO3 were also measured as a function of temperature in air. The Eσ values for conduction were found to be 0.12 and 0.72 eV respectively. The conductivity data correlated well with the catalytic activity of these solid solutions for the oxidation of methane.

Surface characterization and catalytic properties of La1−xAxMO3 perovskite type oxides. Part I. Studies on La0.95Ba0.05MO3 (M = Mn, Fe or Co) oxides.

Abstract Perovskite type La 0.95 Ba 0.05 MO 3 ( M = Mn , Fe or Co ) oxides were prepared by a liquid precursor method and characterized by X-ray diffraction, oxygen desorption, and X-ray photoelectron spectroscopic studies. XPS analysis revealed the characteristic changes of the O1s peak (∼530 eV) resulting from the substitution of Ba in LaMO 3 . The methane deep oxidation reaction was carried out in a plug flow reactor with 2% CH 4 in air in the temperature range of 200 to 700°C. Among the three compounds, the Mn perovskite showed the lowest temperature for the methane conversion with a maximum at 700°C. Temperature programmed desorption (TPD) experiments also indicated the maximum oxygen desorption for Mn perovskite in this temperature range. The results for oxygen chemisorption and surface area of these samples were found to be consistent with the observed variation in the catalytic oxidation of methane.

New electrochemical sensors for oxygen determination

Abstract Oxygen sensors which can operate in wider oxygen pressure and temperature ranges than currently used stabilized zirconia-based oxygen sensors are described. Strontium fluoride-lanthanum fluoride solid solution electrolytes have higher conductivities than zirconia and, when dispersed with an oxide phase, can operate at lower temperatures and oxygen partial pressures than zirconia-based oxygen sensors. Other potential electrolyte materials for low oxygen partial pressure applications are some very stable perovskite oxides, which are pure oxide-ion conductors at lower oxygen partial pressures than stabilized zirconia. In high-temperature aggressive environments, the lifetime of zirconia-based oxygen sensors can be extended through the use of a non-isothermal sensor, in which the temperature of the reference electrode is reduced.

The electrolytic properties of LaYO3 and LaAlO3 doped with alkaline-earth oxides

Abstract The electrolytic properties of two perovskite oxides, LaYO3 and LaAlO3, doped with alkaline-earth cations have been studied. Conductivity measurements show that doped-LaYO3 is a p-type semiconductor at high oxygen partial pressures, but becomes a pure oxide-ion conductor at low oxygen partial pressures. EMF measurements of oxygen concentrations cells confirm that both doped LaAlO3 and doped LaYO3 are pure oxide-ion conductors at low oxygen pressures.

Effect of surface area on CO oxidation by the perovskite catalysts La1−xSr x MO3-δ (M = Co, Cr)

The catalytic oxidation of carbon monoxide by two compounds in the system La0.8 Sr0.2MO3-δ (M = Co, Cr) was measured at different BET surface areas of the oxide materials. The light-off temperature was found to decrease with increasing surface area up to a certain level beyond which it remained constant for the cobalt-containing compound. This limiting value could not be attained for the chromium-containing compound because of equilibration problems in the preparation. As the active catalytic area as distinct from the total (BET) catalyst area was not clarified, these preliminary results show the need for further inquiry.

A fluoride-based composite electrolyte

Abstract A new composite electrolyte, made by dispersing SrO or SrS into a solid solution electrolyte containing 70% mole SrF2-30 % mole LaF3 is described. The appropriate electrochemical cells were assembled and the measured EMFs showed that this kind of composite can function satisfactorily as the electrolyte in oxygen concentration cells when SrO, or La2O3 is dispersed in the electrolyte, and in a sulfur concentration cell when SrS is the dispersed phase.

Solid oxide solutions as catalysts —A comparison with supported Pt

Solid oxide solutions (SOS) of La, Sr and Cr of the formula La1-xSrxCrO3-β (where δ is the deviation from stoichiometry and 0 <x < 0.5) exhibit catalytic activity for oxidation (CO-oxidation) which rivals that of a highly dispersed 0.5% Pt/alumina fume abatement catalysts. Hydrogenation (of propylene) activity is also manifest with the SOS formulation. Exposure of the SOS catalyst to H2S does not reduce catalytic activity.

Thermodynamics of the (lanthanum + strontium + copper + oxygen) high-Tc superconductors I. Heat capacities of SrCuO2, Sr2CuO3, and Sr14Cu24O41

Abstract The subambient heat capacities of three (SrO)x(CuO)y inter-oxide samples with x = y = 1; x = 2, y = 1; and x = 1, y = 2, were measured as a part of an investigation into the chemical thermodynamics of the high-Tc superconducting system (lanthanum + strontium + copper + oxygen). No anomalous contribution to the heat capacity was observed over the experimental region. The heat capacity of the Sr14Cu24O41 phase was derived from that of the mixed phase of nominal composition x = 1, y = 2. All samples exhibit a Debye-like behavior at low temperatures with finite linear electronic terms. Thermophysical properties at selected temperatures are presented and compared for the three compounds. The values of Smo at 298.15 K for the three compounds Sr 1 2 Cu 1 2 O , Sr 2 3 Cu 1 3 O , and Sr 14 38 Cu 24 38 O 41 38 were: 5.828·R, 5.953·R, and 6.032·R, respectively.

Surface characterization and catalytic properties of La1 − xAxMO3 perovskite oxides. Part II. Studies on La1 − xBaxMnO3 (0 ⩽ x ⩽ 0.2) oxides

Abstract Barium substituted lanthanum manganite perovskite-type oxides were prepared by a liquid precursor method and characterized by X-ray diffraction, oxygen desorption, and X-ray photoelectron spectroscopic studies. XPS analysis revealed the significant changes in the doublet characteristics of the Ols peak (∼ 530 eV) resulting from the substitution of Ba in LaMnO 3 . A well defined Ba3d spectra was seen only with more than 10 mol% substitution of Ba in La site. Methane oxidation reaction was carried out in a flow reactor with a feed containing 0.28% CH 4 , 15% oxygen and balance helium. The conversion of methane was followed in the temperature range of 200 to 700 °C. The light-off temperature for 50% methane conversion was found to be the lowest (490 °C) for La 0.9 Ba 0.1 MnO 3 composition. Temperature Programmed Desorption (TPD) experiments of oxygen indicate two maxima centered around 320–550 °C and 650–760 °C respectively. In the low temperature oxygen desorption region, the peak maximum was found to shift with Ba substitution which also corresponded to the activity of methane. The results for oxygen chemisorption and surface composition of these samples are related to the methane oxidation properties.