G. Córdoba

Influence of manganese ions on the anatase–rutile phase transition of TiO2 prepared by the sol–gel process

Abstract The anatase–rutile phase transformation of TiO 2 containing various amounts of Mn 2+ ions was studied by X-ray diffraction, electron spin resonance spectroscopy, and surface area measurements. The samples were prepared by the sol–gel process from titanium(IV) isopropoxide and manganese(II) acetate tetrahydrate. The amorphous powders were calcined for 2 h at different temperatures. Significant structural changes were observed during the various stages of the phase transformation. It was concluded that at low dopant concentrations, manganese ions are incorporated in the TiO 2 structure, and the anatase phase is stabilized, but at larger amounts, part of the dopant is segregated on the surface of TiO 2 and the rutile formation is accelerated. This was clearly confirmed by electron spin resonance. This experiment allowed the authors to follow the manganese oxidation and the phase transformation in detail.

Gas phase hydrogenation of o-xylene over palladium catalysts

Abstract Gas phase hydrogenation of o -xylene over palladium supported catalysts was studied. Catalysts were characterized by hydrogen and carbon monoxide chemisorption and hydrogen-oxygen titrations. The supports used were silica, γ-alumina and silica-alumina. The products of the reaction were cis - and ( trans -1,2-dimethylcyclohexane and -1,2-dimethylcyclohexene. The selectivity was modified by temperature, metal dispersion, and acidity of the support, the production of the trans isomer increasing with an increase in each of these three factors. On the contrary, the presence of electron-donor molecules, such as pyridine, increased the selectivity to the cis isomer. The production off trans -1,2-dimethylcyclohexane may be adequately explained by the ‘roll-over’ of the intermediate species. It is proposed that the electron density of palladium is the determinant factor for the stereoselectivity of the catalysts.

Synthesis, characterization and luminescence properties of Tb3+ and Eu3+-doped poly(acrylic acid)

Abstract Tb 3+ and Eu 3+ -doped poly(acrylic acid) (PAA) was synthesized by polymerization in aqueous solution of the monomer partially neutralized with various amounts of Tb 2 O 3 and Eu 2 O 3 , using potassium persulfate as initiator. The monomer modification was confirmed by 1 H-NMR. The doped polymer was characterized by 1 H-NMR, Fourier transform infrared (FT-IR) and luminescent spectroscopy. The NMR and FT-IR results show clear evidence of the coordination between the dopant cations and the carboxylate anions, which are formed during the partial neutralization of the monomer prior to polymerization. On the other hand, the polymer phosphorescence intensity decreased as the Eu 3+ concentration was increased due to the complex formation. Additionally, the inhomogeneous broadening and the relative intensity of the 5 D 0 → 7 F 1,2 transitions of Eu 3+ ions, and also, the relatively high intensity of 5 D 4 → 7 F 5 green emission of Tb 3+ , indicate that the dopant ions are uniformly distributed in low symmetry coordination sites. Besides, when both dopants are incorporated to the polymer, the Eu 3+ ions quench the luminescence of Tb 3+ , indicating an efficient energy transfer from Tb 3+ to Eu 3+ .

Probing RuPdSiO2 catalysts by gas phase o-xylene hydrogenation

Abstract Ruue5f8Pd SiO 2 catalysts were characterized by hydrogen chemisorption, XPS and TPD of CO, and their catalytic activity was obtained for the gas phase hydrogenation of o-xylene at atmospheric pressure. A small maximum in activity appeared in the bimetallic catalysts at temperatures at which palladium is inactive. The stereoselectivity of the bimetallic catalysts differed markedly from that of monometallic ruthenium. A ligand effect between surface Ru and Pd atoms is suggested to be responsible for the catalytic behaviour of the bimetallic catalysts.

Gas phase hydrogenation of m-xylene on palladium catalysts

Gas phase hydrogenation of m-xylene on palladium supported catalysts was studied and compared with the hydrogenation of o-xylene. The modification in the stereoselectivity of palladium due to an increase in the acidity of the support is explained in terms of an increased residence time of the reactive species on the active surface, leading to a product composition closer to that predicted by thermodynamics.AbstractГазофазное гидрирование м-ксилола на нанесенных палладиевых катализаторах исследовали и сравнивали с гидрированием о-ксилола. Изменения в стереоселективности палладия за счет повышения кислотности носителя объясняют повышенными временами местопребывания реактивных частиц на активной поверхности, приводя к продукту со строением, более близким к предсказанному термодинамикой.

Gas phase hydrogenation of o-xylene on Ru supported catalysts

The gas phase hydrogenation of o-xylene on supported ruthenium catalysts has been studied. Metal dispersity, acidity of the support and carbonaceous residues modified the stereoselectivity of Ru. Following the “rollover” model for the reaction, the results are explained in terms of the residence time of the reactive species on the metal surface.

CO oxidation over CuO/ZrO2 catalysts: effect of loading and incorporation procedure of CuO

Abstract CuO/ZrO2 catalysts were prepared by in situ sol–gel and impregnation methods, with CuO concentrations of 1 and 3·6 mol.-% calcined at 400°C. The catalysts were characterised by S BET, XRD, diffuse reflectance ultraviolet-visible spectroscopy and temperature programmed reduction (H2). The catalytic activity was performed in the CO oxidation. The XRD, diffuse reflectance ultraviolet-visible spectroscopy and temperature programmed reduction results showed that the dispersion of Cu species depends on the method of preparation and on the copper loading. The CuO/ZrO2 catalysts prepared by impregnation procedure showed higher catalytic activity than those prepared by the in situ sol–gel method. This observation can be explained by the presence of small particles of CuO on the catalyst prepared by impregnation (higher dispersion), which does not occur in the catalysts obtained by the in situ sol–gel method due to an important amount of Cu2+ ions that were incorporated into the support network, which are considered catalytically inactive.

Anatase-Rutile Phase Transformation of Cu2+ Doped TiO2 in a Non-Reducing Atmosphere

The anatase-rutile phase transformation of TiO2 containing various Cu2+ amounts of Cu ions was investigated in argon atmosphere by means of X ray diffraction and electron spin resonance spectroscopy. The samples were prepared by the sol-gel method from titanium(IV) isopropoxide and copper(II) nitrate. The amorphous samples were calcined in argon atmosphere for 2 h at different temperatures. In the presence of Cu2+, the anatase-rutile transition temperature is lower than the corresponding one in pure TiO2. The transformation in argon atmosphere changes the transition temperatures due to the spontaneous reduction of Cu2+ and Ti4+ ions to metallic copper and Ti3+ respectively. The broadening of the electron spin resonance spectra from this ion show that Ti3+ ions are present in different sites into the TiO2 lattice. This are confirmed by electron spin resonance and allowed us to follow the phase transformation in detail.