J.L. Boldú

Dehydroxylation and the Crystalline Phases in Sol-Gel Zirconia

The formation and evolution with temperature of the crystalline phases in sol-gel ZrO2 was analyzed by using X-ray powder diffraction, refinement of the crystalline structures, ESR, and UV-Vis spectroscopy. The precursor phase of crystalline zirconia was amorphous Zr(OH)4 with the same local order as the tetragonal crystalline phase. This amorphous phase dehydroxylated with temperature, generating nanocrystalline tetragonal zirconia, and producing point defects that stabilized the tetragonal structure, generated a paramagetic ESR signal with g values like the free electron, and had a light absorption band at 310 nm. When the sample was annealed at higher temperatures, it continued dehydroxilating, and the point defects disappeared, causing the transformation of the nanocrystalline tetragonal phase into nanocrystalline monoclinic zirconia. The two crystalline nanophases coexisted since the beginning of crystallization.

Study of the interaction of cadmium with membrane-bound succinate dehydrogenase

Cadmium ions inhibit membrane-bound succinate dehydrogenase with a second-order rate constant of 10.42 mM−1s−1 at pH 7.35 and 25°C. Succinate and malonate protect the enzyme against cadmium ion inhibition. The protection pattern exerted by succinate and malonate suggests that the group modified by cadmium is located at the active site. The pH curve of inactivation by Cd2+ indicates the involvement of an amino acid residue with pKa of 7.23.

Coke formation on an industrial reforming Pt–Sn/γ‐Al2O3 catalyst

The characterization of the coke deposited on an industrial Pt–Sn/γ‐Al2O3 catalyst, used in a continuous reforming process, was performed with AFM, XRD, FTIR, EPR, NMR, TG‐DTG and DTA techniques. Composition, structure and location of the coke on the catalyst were investigated. The coke was predominantly deposited on the catalyst surface and in the interstices between the catalyst particles. Its content increased along the reactor from top to bottom. Coke was deposited in the form of uniform films and clusters of three‐dimensional disks with diameters between 0.12 and 0.18 μm. It had a pseudo‐graphite structure produced by the dehydrogenation and polymerization of the aromatic precursor compounds. The coked catalyst showed a good combustion behavior; it was regenerated below 550°C. These results are important to elucidate the coke formation mechanism, to generate new continuous reforming catalysts, and to optimize the reactor operation parameters.

Hydrolysis, Condensation, and Tetragonal Phase Formation in Sol-Gel Zirconia Prepared with Electron-Irradiated Alkoxide Solutions

Sol-gel zirconia precursor, zirconium n-butoxide in ter-butanol, was irradiated with 1.3 MeV electrons to a dose of 330 KGy. Gelling was instantaneously produced when an aqueous solution of sulfuric, hydrochloric or acetic acid was added to the irradiated solution; no hydrolysis catalyst was required. Samples were characterized with X-ray powder diffraction, infrared spectroscopy, and electron paramagnetic resonance. The electron irradiation accelerated hydrolysis and condensation, which avoided the stabilization of the tetragonal phase via carboxyls, and decreased the capability of sulfate ions to stabilize it. These results suggest that the stabilization of the tetragonal phase of sol-gel zirconia via carboxyl and sulfate ions depends on their diffusion in the sol.

EPR measurements of the spin-hamiltonian zero-field splitting parameters as a function of temperature in MN2+-doped Cs2NaLaCl6

Abstract The spin-Hamiltonian zero-field splitting parameter b 2 0 is measured as a function of temperature in Mn 2+ doped Cs 2 NaLaCl 6 single crystals (elpasolite type). The experiments showed that the Mn 2+ ion occupies two different tetragonal symmetry sites in the cubic crystalline lattice of Cs 2 NaLaCl 6 . Both sites possess principal Z-axes along 〈100〉 crystallographic directions. Electron spin resonance measurements were made at X-band frequencies (9.2 GHz) in the temperature range from 12.8 to 300 K. The present results are taken into account in order to establish a possible relation between the spin-Hamiltonian parameter b 2 0 and the temperature. A tentative determination of the Debye temperature is derived from the experimental data.

Spectroscopic characterization of sol–gel silica obtained by electron irradiation

In the present work, the gelation of tetraethoxysilane TEOS was carried out using electron irradiation. The electron . source used was a 2 MeV Van de Graaff electron accelerator. Two samples were gelled; for the first one SiO -1 , a solution 2 of TEOS was irradiated with a total dose of 324 kGy. Afterwards, NH OH was added, and the gelation occurred 4 . instantaneously. The second sample SiO -2 , was prepared using a solution of TEOS, but in this case the ammonium 2 hydroxide was added to the solution before irradiation. FTIR spectroscopy results show a significant energy band shift for the pre-hydroxylized TEOS irradiated sample. The presence of room temperature stable OH y species was also established by EPR techniques in the SiO -2 irradiated sample. An unusually high acidity was observed in both samples. q 1999 2

Spectroscopic study of catalysts derived from Li/MgO sol-gel

Abstract Li/MgO catalysts derived from sol-gel materials are shown to enhance the catalytic activity for conversion of methane to ethylene. These catalysts were studied by UV-VIS and EPR spectroscopy. Both characterization methods evidence Li + located in the magnesia matrix.