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High-temperature proton conductors are promising as electrolytes for intermediate-temperature solid oxide fuel cells. Among them, BaCeO 3 -based materials have high proton conductivity but rather poor chemical stability. In contrast, barium zirconates are rather stable, but have poorly reproducible densities and conductivities. In this study, the investigation of BaCe 1ÀxÀy Zr x Y y O 3Àd solid solutions (x ¼ 0, 0.10, 0.20, 0.30, 0.40; y ¼ 0.15, 0.20) was undertaken, with the final aim of finding a composition having both high conductivity and good stability. The influence of the modified sol-gel Pechini synthetic approach on the powder morphology, and of a barium excess on the densification were demonstrated. Single-phase perovskite powders were prepared and high density pellets were obtained at temperatures lower than those commonly employed. Stability tests demonstrated that the Zr introduction into doped barium cerate greatly enhanced the chemical stability, particularly for Zr $ 20%. The proton conductivities, measured in a humidified H 2 /Ar atmosphere by impedance spectroscopy, were only slightly influenced by the Zr amount. Overall, BaCe 1ÀxÀy Zr x Y y O 3Àd solid solutions having Zr z 20-40% and Y z 15-20% showed good chemical stability and high conductivity.

Following our previous papers regarding Pd/C and Pd/SiO2 catalysts, the determination of the Pd/CO average chemisorption stoichiometry from experimental data has been extended to the Pd/alumina system. To this purpose, the Pd crystallite size determined by the well-established line-broadening (LB) method, using X-ray powder diffraction (XRPD) associated with the Fourier analysis of suitable best-fitted peak profiles, and the chemisorbed CO volume, determined by the pulse flow method, were employed. HRTEM was also used to check the structural features of the Pd particles, which proved to be monodomains of cubooctahedral shape. A high-dispersion 5 wt % Pd/alumina catalyst was prepared by impregnation and thermally treated at different temperatures up to 1073 K. Even at this high temperature a good Pd dispersion was retained. A suitable subtraction of the X-ray scattering, due to the support, was done to obtain the real profile of the supported metal, from which the surface-weighted average particle diameters were calculated. Using these values and the corresponding CO chemisorbed volumes, a Pd/CO average chemisorption stoichiometry close to 2 was obtained. On the basis also of our previous work regarding Pd/C and Pd/SiO2 catalysts, it can be concluded that for all supported Pd catalysts such stoichiometry is close to 2, independent of support nature and Pd dispersion, when chemisorbed volumes are measured by the routinely used pulse flow method.

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The convolutive X-ray peak profile-fitting methodology described in the previous paper [Enzo, Fagherazzi, Benedetti & Polizzi (1988). J. Appl. Cryst. 21,[536][537][538][539][540][541][542] has been applied to a series of fluorite samples milled for different times and two zirconia ultrafine powders, by using either pseudo-Voigt or Voigt (in the Kielkopf approximation) functions, in order to investigate the broadening due to microstructural factors (crystallite size and lattice distortions). In the fluorite milled powders Fourier analysis (Warren & Averbach and Vogel, Haase & Hosemann methods) has shown that lattice disorder is due to dislocations. Values for the crystallite size as well as for the lattice distortion parameters similar to those obtained with Fourier methods have been found using a suitable integral breadth plot based on an assumed Cauchyian shape for the broadened profiles. A very high computability limit was reached for the crystallite size (about 5000A). In the ultrafine powders of zirconia the cubic, tetragonal and monoclinic polymorphs were studied. The noticeable Gaussian content present in the 222 reflections for both cubic and tetragonal forms can be explained in terms of the presence of'intercrystalline'-like distortions. This kind of lattice distortion, detected by means of the Fourier analysis, has also been confirmed by 'simplified' methods (two-line and single-line procedures). Examples of limitations of the present methodology are reported and discussed.