A Study of Cerium–Manganese Mixed Oxides for Oxidation Catalysis

Cerium–manganese mixed oxides with compositions of Ce0.5Mn0.5O1.75 and Ce0.8Mn0.2O1.9 were prepared by the citric-acid (Pechini) method and their catalytic properties were compared to CeO2 and Mn2O3. The mixed oxides exhibited higher specific rates than either CeO2 or Mn2O3 for oxidation of both methane and n-butane. While XRD measurements of the mixed oxides suggested that the materials had primarily the fluorite structure, oxygen isotherms, measured by coulometric titration at 973 K, exhibited steps associated with MnO–Mn3O4 and Mn3O4–Mn2O3 equilibria, implying that manganese oxide must exist as separate phases in the solids. The P(O2) for the MnO–Mn3O4 equilibrium is shifted to lower values in the mixed oxides, indicating that the manganese-oxide phase is affected by interactions with ceria

Dynamic Changes in LSM Nanoparticles on YSZ: A Model System for Non-Stationary SOFC Cathode Behavior

The interaction between nanoparticles of strontium-doped lanthanum manganite (LSM) and single-crystal yttria-stabilized zirconia (YSZ) was investigated using atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX). Nanoparticles of LSM were deposited directly onto single-crystal YSZ (100) substrates using an ultrasonic spray nozzle. As samples were annealed from 850 to 1250 degrees C, nanoparticles gradually decreased in height and eventually disappeared completely. Subsequent reduction in H-2/H2O at 700 degrees C resulted in the reappearance of nanoparticles. Studies were carried out on identical regions of the sample, allowing the same nanoparticles to be characterized at different temperatures. Morphological changes indicate the formation of a thin layer of LSM, and XPS results support the observation by indicating an increase in signal from the La and Sr and a decrease in signal from the Y and Zr with increasing temperature. SEM/EDX was used to verify that the nanoparticles in the reduced sample contained La. The changes in the LSM/YSZ morphology may be important in explaining the nonstationary behavior observed in operating solid-oxide fuel cells (SOFCs). The thin layer of LSM initially results in poor cathode performance; reducing conditions then lead to film disruptions, indicating nano/microporosity, that increase oxygen ion diffusion and performance

Oxidation entropies and enthalpies of ceria–zirconia solid solutions

The thermodynamic redox properties for a series of ceria–zirconia solid solutions have been measured by determining their oxidation isotherms between 873 and 1073 K. Isotherms were obtained using Coulometric titration and using O2 titration of samples equilibrated in flowing mixtures of H2 and H2O. Samples having the following compositions were studied after calcinations at 973 and 1323 K: CeO2, Ce0.92Zr0.08O2, Ce0.81Zr0.19O2, Ce0.59Zr0.41O2, Ce0.50Zr0.50O2, Ce0.25Zr0.75O2, Ce0.14Zr0.86O2, and ZrO2. While the oxidation enthalpy for CeO2 was between −750 and −800 kJ/mol O2, the oxidation enthalpies for each of the solid solutions were between −500 and −550 kJ/mol O2 and essentially independent of the extent of reduction. The shapes of the isotherms for the solid solutions were affected by the oxidation entropies, which depended strongly on the sample composition and the extent of reduction. With CeO2, Ce0.92Zr0.08O2, and Ce0.14Zr0.86O2, the samples remained single-phase after calcination at 1323 K and the thermodynamic redox properties were unaffected. By contrast, Ce0.59Zr0.41O2 formed two phases following calcination at 1323 K, Ce0.78Zr0.22O2 (71 wt.%) and Ce0.13Zr0.87O2 (29 wt.%); the isotherm changed to that which would be expected for a physical mixture of the two phases. A model is presented which views reduction of the solid solutions in terms of the local atomic structure, with the formation of pyrochlore-like clusters causing the increased reducibility of the solid solutions. Some of the changes in reducibility are associated with the number of sites from which oxygen can be removed in order to form pyrochlore-like clusters

Oxidation Enthalpies for Reduction of Ceria Surfaces

The thermodynamic properties of surface ceria were investigated through equilibrium isotherms determined by flow-titration and coulometric-titration measurements on high-surface-area ceria and ceria supported on La-modified alumina (LA). While the surface area of pure ceria was found to be unstable under redox conditions, the extent of reduction at 873 K and a P(O2) of 1.6x10-26 atm increased with surface area. Because ceria/LA samples were stable, equilibrium isotherms were determined between 873 and 973 K on a 30-wt% ceria sample. Oxidation enthalpies on ceria/LA were found to vary with the extent of reduction, ranging from -500 kJ/mol O2 at low extents of reduction to near the bulk value of -760 kJ/mol O2 at higher extents. To determine whether +3 dopants could affect the oxidation enthalpies for ceria, isotherms were measured for Sm+3-doped ceria (SDC) and Y+3-doped ceria. These dopants were found to remove the phase transition observed in pure ceria below 973 K but appeared to have minimal effect on the oxidation enthalpies. Implications of these results for catalytic applications of ceria are discussed

Impedance Analysis of Electrochemical NOx Sensor Using a Au/Yttria-Stabilized Zirconia (YSZ)/Au cell

An electrochemical cell employing a YSZ electrolyte and two Au electrodes was utilized as a model system for investigating the mechanisms responsible for impedance metric NO{sub x} (NO and NO{sub 2}) sensing. The cell consists of two dense Au electrodes on top of a porous/dense YSZ bilayer structure (with the additional porous layer present only under the Au electrodes). Both electrodes were co-located on the same side of the cell, resulting in an in-plane geometry for the current path. The porous YSZ appears to extend the triple phase boundary and allows for enhanced NO{sub x} sensing performance, although the exact role of the porous layer is not completely understood. Impedance data were obtained over the frequency range of 0.1 Hz to 1 MHz, and over a range of oxygen (2 to 18.9%) and NO{sub x} (10 to 100 ppm) concentrations, and temperatures (600 to 700 C). Data were fit with an equivalent circuit, and the values of the circuit elements were obtained for different concentrations and temperatures. Changes in a single low-frequency arc were found to correlate with concentration changes, and to be temperature dependent. In the absence of NO{sub x}, the effect of O{sub 2} on the low-frequency resistance could be described by a power law, and the temperature dependence described by a single apparent activation energy at all O{sub 2} concentrations. When both O{sub 2} and NO{sub x} were present, however, the power law exponent varied as a function of both temperature and concentration, and the apparent activation energy also showed dual dependence. Adsorption mechanisms are discussed as possibilities for the rate-limiting steps