Structural characterization of nanosized CeO(2)-SiO(2), CeO(2)-TiO(2), and CeO(2)-ZrO(2) catalysts by XRD, Raman, and HREM techniques

Structural characteristics of nanosized ceria-silica, ceria-titania, and ceria-zirconia mixed oxide catalysts have been investigated using X-ray diffraction (XRD), Raman spectroscopy, BET surface area, thermogravimetry, and high-resolution transmission electron microscopy (HREM). The effect of support oxides on the crystal modification of ceria cubic lattice was mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahighly dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO(2)-SiO(2) sample primarily consists of nanocrystalline CeO(2) on the amorphous SiO(2) surface. Both crystalline CeO(2) and TiO(2) anatase phases were noted in the case of CeO(2)-TiO(2) sample. Formation of cubic Ce(0.75)Zr(0.25)O(2) and Ce(0.6)Zr(0.4)O(2) (at 1073 K) were observed in the case of the CeO(2)-ZrO(2) sample. Raman measurements disclose the fluorite structure of ceria and the presence of oxygen vacancies/Ce(3+). The HREM results reveal well-dispersed CeO(2) nanocrystals over the amorphous SiO(2) matrix in the cases of CeO(2)-SiO(2), isolated CeO(2), and TiO(2) (anatase) nanocrystals, some overlapping regions in the case of CeO(2)-TiO(2), and nanosized CeO(2) and Ce-Zr oxides in the case of CeO(2)-ZrO(2) sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO(2) is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of the mixed oxide systems is more than that of pure CeO(2) and is system dependent.     

Brownmillerites CaFeO2.5 and SrFeO2.5 as Catalyst Support for CO Oxidation

The support material can play an important role in oxidation catalysis, notably for CO oxidation. Here, we study two materials of the Brownmillerite family, CaFeO_2.5 and SrFeO_2.5, as one example of a stoichiometric phase (CaFeO_2.5, CFO) and one existing in different modifications (SrFeO_2.75, SrFeO_2.875 and SrFeO₃, SFO). The two materials are synthesized using two synthesis methods, one bottom-up approach via a complexation route and one top-down method (electric arc fusion), allowing to study the impact of the specific surface area on the oxygen mobility and catalytic performance. CO oxidation on ¹⁸O-exchanged materials shows that oxygen from SFO participates in the reaction as soon as the reaction starts, while for CFO, this onset takes place 185 °C after reaction onset. This indicates that the structure of the support material has an impact on the catalytic performance. We report here on significant differences in the catalytic activity linked to long-term stability of CFO and SFO, which is an important parameter not only for possible applications, but equally to better understand the mechanism of the catalytic activity itself.     

Peroxo and alkylperoxidic molybdenum(VI) complexes as intermediates in the epoxidation of olefins by alkyl hydroperoxides

Novel oxoperoxomolybdenum(VI) complexes with the general formula MoO(O₂)L₂X₂ (III, L = DMF, HMPT) and MoO(O₂)Cl(ON)L(IV, ON) = pyridin-2-carboxylate (Pic), 8-hydroxyquinolinate (Quin) were prepared from the reaction of Ph₃COOH or H₂O₂ with the corresponding cis-dioxo complexes. In the reaction with Ph₃COOH both oxygen atoms of the peroxo moiety were found, by ¹⁸O labeling experiments, to come from the hydroperoxide. The X-ray crystal structure of MoO(O₂)Cl(Pic)(HMPT) revealed a bipyramidal pentagonal surounding with a rather short OO distance (1.41 Å). Complexes III were found to be more reactive than MoO(O₂)₂,HMPT for the epoxidation of olefins (oxidative cleavage products are consecutively formed) but react by the same cyclic peroxymetalation mechanism. The absence of reaction in the case of complexes IV illustrates the necessity for the metal to possess an equatorial releasable coordination site adjacent to the peroxo group for the oxygen transfer to occur. Catalytic oxidation of olefins using Ph₃COOH gave a selectivity in oxygenated products very different from that using t-BuOOH, and ¹⁸O labeling studies showed that alkyl-peroxidic rather than peroxo species are intermediates in this latter reaction. The mechanism of epoxidation of olefins by alkyl hydroperoxides catalyzed by d⁰ metal complexes is discussed.     

Properties of a triazolopyridine system as a molecular chemosensor for metal ions, anions, and amino acids

The characteristics as a chemosensor of the compound 3-methyl-6,8-di(2-pyridyl)-[1,2,3]triazolo[5',1':6,1]pyrido[2,3-]pyrimidine (1) have been analyzed. Interaction with Cu(2+) produces a quenching of the fluorescence, while interaction with Zn(2+) leads to a quenching of the fluorescence followed by a bathochromic shift. The crystal structure of the Zn(1)(H(2)O)(3)(ClO(4))(2) x H(2)O complex shows the coordination of Zn(2+) through the terpyridine moiety. The octahedral site is completed by three water molecules. Interactions of the Zn(2+) complex with the anions sulfate, nitrate, nitrite, and dihydrogenphosphate in ethanol produce hypsochromic shifts and restoration of the fluorescence whose magnitude depends on the anion involved. The maximum interaction is observed for H(2)PO(4)(-). Interactions of the Zn(2+) complex with the amino acids l-aspartate and l-glutamate have also been explored showing a higher interaction with l-aspartate.     

Inversion de la transformation de Pompeiu locale dans les espaces hyperboliques reel et complexe: Cas des deux boules

In this paper we extend the result we have established for the hyperbolic disk in [8] to the real and complex hyperbolic spaces. This includes the reconstruction of a function defined in a fixed ball from its averages on balls of radiir ₁,r ₂ lying inB(0,R).