Nanocrystalline alumina was synthesized by using the sol−gel method with aluminum sec-butoxide as the precursor. FTIR, TG, XRD, and crystalline structure refinement were used to analyze the phase transformations and their crystalline structures. When samples were annealed at 200 °C, they had two nanocrystalline phases γ-boehmite and γ*-boehmite, which had the same crystalline structure but different lattice parameters and defect numbers in the structure. After sample annealing above 400 °C, the γ-boehmite and γ*-boehmite phases were transformed into nanocrystalline γ-Al2O3 and θ-Al2O3. The γ-alumina crystalline structure contained hydroxyls that brought about cationic defects in aluminum sites, mainly in those of octahedral symmetry. In the 27Al NMR spectra of the samples calcined at 400, 600, and 800 °C, a chemical shift around 33 ppm was observed, which occurred between that of aluminum in oxygen octahedra and aluminum in oxygen tetrahedra. Its origin can be explained by assuming the existence the substitution of some lattice oxygen ions in the octahedral symmetry by hydroxyl groups.

In the sol-gel Cu/TiO 2 system, nanocrystalline brookite, which was stable even after annealing the sample at 400 °C, was generated in high concentrations. This was achieved with copper chloride as copper precursor. The respective samples had a light absorption band at 562 nm that corresponded to an energy bandgap of 2.30 eV, which is smaller than the values reported for titania. When copper sulfate was the precursor, brookite was absent, but nanocrystalline anatase, the main phase, was stabilized even at 800 °C by sulfate ions. After annealing the samples at low temperatures, copper was well dispersed for 1 wt % copper, but part of it segregated forming copper compounds when the copper concentration was 10 wt %. By annealing the samples at 800 °C, copper oxidized to CuO; except in the 10 wt %-copper sample prepared with copper sulfate, where it formed antlerite.

No abstract

Magnesium−alumina mixed oxide catalysts at Mg:Al atomic ratios of 3:1, 2:1.8, and 1:2.8 were synthesized by using the sol−gel technique. Dehydroxylation and phase transformations were studied with thermogravimetry. Crystalline structures were measured with X-ray powder diffraction. Position and concentration of anionic and cationic vacancies were obtained by refining crystalline structures with the Rietveld technique. When samples were annealed below 400 °C, boehmite, brucite, hydrotalcite, and glushinskite were formed. When they calcined at 600 °C, boehmite was transformed into γ-Al2O3, and brucite, hydrotalcite, and glushinskite into periclase. In magnesium-rich samples, magnesium ions were incorporated into the γ-Al2O3 lattice, expanding its unit cell. Sample dehydroxylation produced oxygen vacancies in boehmite, brucite, and θ-Al2O3. Periclase and γ-Al2O3, however, had cationic vacancies in concentrations that depended on calcining temperature. Different models are proposed for explaining the formation mechanisms of the anionic and cationic vacancies.

2-Propanol decomposition at 200 °C on magnesium oxide prepared by the hydrolysis of magnesium ethoxide (sol−gel method) is reported. With hydrochloride acid, acetic acid, or ammonia as hydrolysis alkoxide catalysts, the acidity and the selectivity to propene of the catalysts follow the sequence, MgO (HCl) > MgO (acetic acid) > MgO (ammonia). In contrast, the basicity of the samples, defined as the ratio dehydrogenation rate/dehydration rate, is found to be depending on the particle size of the catalysts. The basicity trend found is MgO (acetic acid) > MgO (ammonia) > MgO (HCl) and it correlates well with the particle size determined by the Rietveld refinement.