The mixed ionic-electronic conductor (MIEC) (Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 )O 3-δ (BSCF) is a renowned material with applications in membrane reactors and as cathodes in solid-oxide fuel cells. Despite BSCF's large oxygen permeabilities, long-time phase instability at intermediate temperatures has been reported. However, the mechanism of this decomposition is still unclear. Here, we present a study of the synthesis of BSCF and compare our results with those obtained from long-time decomposition. Rietveld and Le Bail analysis as well as transmission electron microscopy studies were applied to investigate the reaction sequence in BSCF formation. We are now able to draw the following conclusion about the reaction mechanism: the formation as well as decomposition is due to a reversible reordering of the hexagonal AO 3 -layer stacking sequence in the cubic perovskite, which can occur if the cubic BSCF is kept at temperatures below T ) 1173 K for long time periods, thereby leading to the decomposition of BSCF into a three-phase mixture. The driving force for this reaction was identified to occur at the cobalt site because cobalt prefers a low-spin configuration in the 3+ oxidation state. This reaction occurs only at temperatures below T ) 1173 K because of the oxidation of cobalt at low temperatures.

The effect of the degree of inversion on the electronic transitions and the Raman scattering of ZnFe2O4 is revealed.

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A well crystallized sample of U2Al3C4 was obtained by melting the elemental components in a carbon crucible in a high frequency furnace. The crystal structure of this compound was determined from single-crystal diffractometer data of a twinned crystal: P63mc, a = 342.2(1) pm. c = 2323.0(3) pm. Z = 2 , R = 0.030 for 537 structure factors and 18 variable parameters. The structure can also be described in the higher symmetry space group P63/mmc with one split aluminum position. It consists of close packed layers of uranium and aluminum atoms with carbon atoms at interstitial sites. The structure is closely related to that of Al4C3, which was refined from single-crystal X-ray data to a residual of R = 0.033 for 135 F-values and 11 variables. The hydrolysis of U2Al3C4 with diluted hydrochloric acid resulted in about 74 (wt-)% methane, 8% ethane and ethylene, and 18% saturated and unsaturated higher hydrocarbons.

Mullite-type Bi2Fe4O9 was synthesized using a polyol-mediated method. X-ray powder diffraction (XRD) revealed that the as-synthesized sample is nanocrystalline. It transformed into a rhombohedral perovskite-type BiFeO3 followed by a second transformation into mullite-type Bi2Fe4O9 during heating. Each structural feature, from as-synthesized into crystalline phase, was monitored through temperature-dependent XRD in situ. The locally resolved high resolution transmission electron micrographs revealed that the surface of some heated samples is covered by 4–13 nm sized particles which were identified from the lattice fringes as crystalline Bi2Fe4O9. XRD and Raman spectra demonstrate that the nucleation of both BiFeO3 and Bi2Fe4O9 might simultaneously commence; however, their growth and ratios are dependent on temperature. The diffuse UV/vis reflectance spectra showed fundamental absorption edges between 1.80(1) and 2.75(3) eV. A comparative study between the “derivation of absorption spectrum fitting method” (DASF) and the Tauc method suggests Bi2Fe4O9 to be a direct band gap semiconductor.

Inspired by model studies under ultrahigh vacuum (UHV) conditions, inverse monolithic gold/ceria catalysts are prepared using thermal decomposition of a cerium nitrate precursor on a nanoporous gold (npAu) substrate. Cerium oxide deposits throughout the porous gold material (pores and ligaments 30−40 nm) are formed. npAu disks and coatings were prepared with loadings of about 3 to 10 atom % of ceria. The composite material was tested for the water−gas shift (WGS) reaction (H 2 O + CO → H 2 + CO 2 ) in a continuous flow reactor at ambient pressure conditions. Formation of CO 2 was observed at temperatures as low as 135°C with excellent stability and reproducibility up to temperatures of 535°C. The considerably increased thermal stability of the material can be linked to the presence of metal oxide deposits on the nanosized gold ligaments. The loss of activity after about 15 h of catalytic conversion with heating to 535°C was only about 10%. Photoemission spectroscopy indicates a defect (Ce 3+ ) concentration of about 70% on the surface of the cerium oxide deposits, prior to and after WGS reaction. Raman spectroscopic characterization of the material revealed that the bulk of the oxide is reoxidized during reaction. ■ INTRODUCTIONDuring the last decades increasing demand for a novel type of water−gas shift (WGS) catalyst in the context of mobile and green energy harvesting such as in fuel cells surfaced. 1,2 For either low-temperature fuel cells (polymer electrolyte membrane fuel cells, PEMFC) or high-temperature fuel cells in mobile applications such as in cars a novel type of WGS catalyst is required. These catalysts need to be highly active at low temperatures, shifting CO almost quantitatively to hydrogen (<10 ppm for PEMFCs). They need to be safe, nonpyrophoric and oxidation resistant upon exposure to air (excluding, thus, traditional WGS catalysts), highly durable and long-lasting and have to withstand fast deactivation and shut down procedures. Last but not least they have to be readily applicable into small scale reactor designs, ideally as coatings or monolithic catalyst beds. The focus of this latest surge in research regarding WGS catalysts are precious metal based catalysts. 1,3Among possible candidates, gold is of particular interest as it is cheaper than, e.g., platinum and also a very selective catalyst for the oxidation of CO in the presence of H 2 , making it an ideal catalyst material for procession of hydrogen gas for fuel cells. 4 Recently, Au-CeO 2 nanomaterials have been reported to be very efficient catalysts for the WGS reaction. 2 The interplay between the oxide and the Au provides reactivity toward the dissociation of water. Similarly, as reported for oxidation reactions using molecular oxygen, the synergy between the two partners is critical for the catalytic activity. 5,6 Oxygen vacancies existing in the oxide nanoparticles are suggested to play a key role for the dissociation of water. 7 A blend of catalysts consisting of metal nanoparticles dispersed on an oxide support has been report...

A novel neutral and cation-free LTA-type AlPO(4) membrane has been prepared on porous asymmetric ceramic supports. Hydrogen can be effectively separated from other gases by molecular sieving.