Gold nanoparticles supported on different oxides (SiO 2 , CeO 2 and TiO 2 ) were prepared by the SMAD (solvated metal atom dispersion) and deposition-precipitation (DP) techniques. The physical and chemical characterization of the catalysts was performed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and the catalytic activity was tested during the reaction of low temperature CO oxidation. The structural and surface analyses evidenced the presence of small gold crystallites (cluster size ∼2-5 nm) in all the SMAD-prepared samples and oxidized gold species in the case of the DP catalysts. A different surface distribution of ionic gold species was found on the different supports. By comparing the catalytic activities of the samples, the presence of Au +1 species seems to be the main requisite for the achievement of the highest CO conversion at the lowest temperature. The higher activity of Au/CeO 2 (DP) catalysts at T ≈ 250 K can be ascribed to a better stabilization of the AuO − species by the cerium oxide. Nanosized metallic gold particles exhibit a worse catalytic performance, both on 'reducible' and 'inert' supports, being significantly active only in the temperature range: 400-600 K.
Yttrium-doped barium zirconate (BZY) is the most promising candidate for proton-conducting ceramics and has been extensively studied in recent years. The detailed features of the crystal structure, both short-range and long-range, as well as the crystal chemistry driving the doping process, are largely unknown. We use very high resolution X-ray diffraction (HR-XRD) to resolve the crystal structure, which is very slightly tetragonally distorted in BZY, while the local environment around Zr4+ and Y3+ is probed with extended X-ray absorption fine structure (EXAFS), and the symmetry and vibrations are investigated by using Raman spectroscopy. It is found that barium zirconate shows some degree of local deviation from the cubic arystotype even if undoped, which upon substitution by the perceptibly larger Y3+, playing the role of a rigid inclusion, is further increased. This distortion is one limiting factor concerning the Y3+ solubility. The effects are correlated to the proton conduction properties of BZY
The long-range and short-range structure of nanocrystalline and microcrystalline acceptor-doped ceria is investigated by a combined approach using EXAFS, XANES, Raman, and XRD, and correlated with the oxide-ion conductivity in the bulk and in grain boundaries. Compared to Yb 3+ and Er 3+ , the positive influence of Sm 3+ is attributed to the ability to repel oxygen vacancies, and to keep a localized disorder around the dopant. The long-range structural analysis shows lattice contraction for Yband Er-doping and lattice expansion for Sm-doping. The short-range analysis around the dopants and cerium highlights that a more complex structural rearrangement has to be assumed to explain the complementary results of the different techniques. Nominally trivalent dopants are also shown to have an effect on the electronic structure of ceria, and the consequences on oxide-ion conductivity are highlighted.
The solid solution series Ba(In,Ce)O 3-δ has been investigated with respect to structure, formation, and mobility of protonic defects. Compared to the limited solubility of Y 2 O 3 in BaCeO 3 and BaZrO 3 , the complete solubility of In 2 O 3 is suggested to reflect a relation between absolute hardness of the dopant and the ease of insertion into the hosting lattices. Extended X-ray absorption fine structure (EXAFS) was used to probe the local environment of In 3+ in barium cerate: in the surroundings of the dopant, the orthorhombic structure is strongly modified, resulting in an increase of local symmetry. The InO 6 octahedra are very regular, and there is no indication for any defect clustering. This is suggested to be the main reason for the low entropy of formation of protonic defects by water dissolution. The mobility of such defects is slightly lower than in Y-doped BaCeO 3 , but at high dopant levels the high local symmetry allows for formation of very high concentrations of protonic defects. This leads to high proton conductivities, which render In 3+ an attractive dopant for BaCeO 3 -based proton conductors.
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