T. López

Thermal stability of 12-tungstophosphoric acid supported on zirconia

A freshly precipitated zirconia was impregnated with an ethanol solution of H3[W12PO40]6H2O (TPA) in order to obtain 0‐25 wt.% TPA/ZrO2. The solids, heat-treated from 373 to 1073 K, were examined by means of differential thermal analysis, X-ray diffraction, surface area, infrared, Raman, 31 P nuclear magnetic resonance analyses, and used as catalysts in the decomposition of isopropanol. Below 673 K, TPA on ZrO2 exist as distorted intact Keggin species interacting with Zr‐OH or Zr C groups. However, at 773 K, bulk-like intact TPA species were detected. Above 773 K, the Keggin structure of TPA collapse and transforms into WO3 and phosphorous oxides. The characteristic diffraction peaks of TPA cannot be observed even with 20 wt.% TPA loading, indicating a high dispersion or a very small crystal size of TPA, while 20 wt.% TPA loaded on SiO2 clearly shows the presence of TPA particles.The heat-treatment temperature affects the catalytic activity of TPA on Zirconia. The reaction rate was 3.3 times higher in a TPA/ZrO2 calcined at 773 than that at 473 K. ©2000 Elsevier Science B.V. All rights reserved.

On the antioxidant properties of kynurenic acid: Free radical scavenging activity and inhibition of oxidative stress

Kynurenic acid (KYNA) is an endogenous metabolite of the kynurenine pathway for tryptophan degradation and an antagonist of both N-methyl-D-aspartate (NMDA) and alpha-7 nicotinic acetylcholine (α7nACh) receptors. KYNA has also been shown to scavenge hydroxyl radicals (OH) under controlled conditions of free radical production. In this work we evaluated the ability of KYNA to scavenge superoxide anion (O(2)(-)) and peroxynitrite (ONOO(-)). The scavenging ability of KYNA (expressed as IC(50) values) was as follows: OH=O(2)(-)>ONOO(-). In parallel, the antiperoxidative and scavenging capacities of KYNA (0-150 μM) were tested in cerebellum and forebrain homogenates exposed to 5 μM FeSO(4) and 2.5 mM 3-nitropropionic acid (3-NPA). Both FeSO(4) and 3-NPA increased lipid peroxidation (LP) and ROS formation in a significant manner in these preparations, whereas KYNA significantly reduced these markers. Reactive oxygen species (ROS) formation were determined in the presence of FeSO(4) and/or KYNA (0-100 μM), both at intra and extracellular levels. An increase in ROS formation was induced by FeSO(4) in forebrain and cerebellum in a time-dependent manner, and KYNA reduced this effect in a concentration-dependent manner. To further know whether the effect of KYNA on oxidative stress is independent of NMDA and nicotinic receptors, we also tested KYNA (0-100 μM) in a biological preparation free of these receptors - defolliculated Xenopus laevis oocytes - incubated with FeSO(4) for 1 h. A 3-fold increase in LP and a 2-fold increase in ROS formation were seen after exposure to FeSO(4), whereas KYNA attenuated these effects in a concentration-dependent manner. In addition, the in vivo formation of OH evoked by an acute infusion of FeSO(4) (100 μM) in the rat striatum was estimated by microdialysis and challenged by a topic infusion of KYNA (1 μM). FeSO(4) increased the striatal OH production, while KYNA mitigated this effect. Altogether, these data strongly suggest that KYNA, in addition to be a well-known antagonist acting on nicotinic and NMDA receptors, can be considered as a potential endogenous antioxidant.

FTIR and UV-Vis (diffuse reflectance) spectroscopic characterization of TiO2 sol-gel

Abstract The semiconductive properties of titania are determined by the density of point defects in the crystalline structure. The synthesis method (sol-gel) provides pure and reproducible solids. This technique is very sensitive to hydrolysis catalysts, pH and calcination temperature. In this work, each one of these parameters is systematically varied and correlations with the structures of the obtained solids are established. It is shown that very acidic or basic catalysts are selective to rutile (900 °C). Acid catalysts favour an hydrolysis reaction, but pH does not change the shape or the size of the particles. The reaction mechanism is a nucleophilic substitution SN 2 in the basic medium and most intermediates in the sol-gel reaction are Ti-OH. The characterization techniques are FTIR, UV-Vis (diffuse reflectance), X-ray diffraction, DTA-TGA and scanning electron microscopy.

Effect of the preparation method on the band gap of titania and platinum-titania sol-gel materials

TiO2 and PtTiO2 sol-gel materials were synthesized at pH 3 and 9 by the sol-gel process using titanium alkoxide as reactant. The resultant solids were studied by UV-Vis (diffuse reflectance) spectroscopy. An evaluation of the band gap (Eg) was done in samples thermally treated at different temperatures. VO2−+ vacancies are formed in titania and platinum-titania samples by dehydroxylation when the samples are prepared at pH 3 and by oxygen lattice desorption if the samples are prepared at pH 9. Platinum metal acts as an electron trap.

Raman study of nanosized titania prepared by sol–gel route

Abstract TiO 2 polycrystalline powders have been prepared by different sol–gel routes and have been investigated by Raman spectroscopy and transmission electron microscopy. The influence of different catalysts on the crystalline phases obtained by thermal annealing is studied. The mechanisms that determine the Raman line shapes are discussed, including phonon confinement and pressure effects. The analysis of the Raman spectra, after thermal treatments in air and N 2 , indicates the importance of the non-stoichiometry due to oxygen vacancies.

Characterizations of the thermal decomposition of brucite prepared by sol–gel technique for synthesis of nanocrystalline MgO

Abstract Brucite was synthesized by the sol–gel technique using magnesium diethoxide in a homogeneous reaction medium. Its decomposition, including dehydroxylation, crystallization and phase transformation was studied by XRD, TGA–DTG, CTEM, HRTEM, SAED and FTIR techniques. It has been proposed that during the dehydroxylation and decomposition of the magnesium diethoxide gel, some vacancies are generated. The decomposition of brucite with hexagonal structure to form periclase with the cubic structure occurs in the temperature range from 473 to 773 K. In this case, nanocrystalline MgO is formed from the break down of the brucite crystalline structure.

Preparation of sol-gel sulfated ZrO2SiO2 and characterization of its surface acidity

Abstract Zr Si mixed oxides containing 1.3 to 6.7 wt.-% Zr have been prepared by a sol-gel technique, using tetraethoxysilane and zirconium acetate as precursors. Zirconium is incorporated into the lattice of silica, but can easily be extracted either by self steaming or by acid treatment. The solids obtained in basic medium show a low surface area and low acidity. The samples prepared in acidic medium show large surface areas between 410 and 740 m 2 /g and can be converted to strong acids by contact with a solution of sulfuric acid. They retain pyridine up to 773 K and contain a total number of acid sites comparable to that of HY zeolites i.e. 1.7 meq/g. The catalytic activity for the bifunctional isomerization of n-hexane expressed per gram of zirconium is comparable for the sample containing 4wt.-% Zr and sulfated zirconia.

Effects of structural defects and acid-basic properties on the activity and selectivity of isopropanol decomposition on nanocrystallite sol-gel alumina catalyst

The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transform infrared FTIR spectroscopy of pyridine adsorption and temperature-programmed desorption of CO and NH . The number of acid and 23 basic sites on the samples varied with the calcination temperatures of the samples. The populations of the three different aluminum ions—tetrahedral, pentacoordinated and octahedral, which were identified by the 27 Al MAS NMR, were strongly affected by the sample calcination temperature and the crystalline composition. In the reaction temperature range between 100 and 2508C, isopropanol decomposition on sol-gel alumina catalysts was carried out. It was found that isopropanol decomposition on alumina catalyst was a structural-defect sensitive reaction. The dehydrogenation selectivity to acetone depended on the surface basic sites and the concentration of aluminum vacancies in the crystalline structure of g-Al O. 23 Bimolecular reaction to isopropylether was largely governed by the pentacoordinated aluminum ions which were related to the coordinately unsaturated aluminum ions. A mechanism for the formation of isopropylether was proposed: oxygen . q vacancies were suggested to involve the adsorption step of isopropanol, an intermediate species, CH HC , reacted with 32

Study of the stabilization of zinc phthalocyanine in sol-gel TiO2 for photodynamic therapy applications

Photodynamic therapy (PDT) has emerged as an alternative and promising noninvasive treatment for cancer. It is a two-step procedure that uses a combination of molecular oxygen, visible light, and photosensitizer (PS) agents; phthalocyanine (Pc) was supported over titanium oxide but has not yet been used for cell inactivation. Zinc phthalocyanine (ZnPc) molecules were incorporated into the porous network of titanium dioxide (TiO(2)) using the sol-gel method. It was prepared from stock solutions of ZnPc and TiO(2). ZnPc-TiO(2) was tested with four cancer cell lines. The characterization of supported ZnPc showed that phthalocyanine is linked by the N-pyrrole to the support and is stable up to 250°C, leading to testing for PDT. The preferential localization in target organelles such as mitochondria or lysosomes could determine the cell death mechanism after PDT. The results suggest that nanoparticulated TiO(2) sensitized with ZnPc is an excellent candidate as sensitizer in PDT against cancer and infectious diseases.

Ru/SiO2-impregnated and sol-gel-prepared catalysts: synthesis, characterization, and catalytic properties

Impregnation and sol-gel preparation methods are compared. Only when prepared by sol-gel synthesis, ruthenium is incorporated in the silica network; RuO2 particles are observed, but most Ru is occluded in the silica network. The ruthenium migrates to the surface with thermal treatments and small particles are formed. In the case of impregnated catalysts only RuO2 particles are observed. Their diameter is larger when the catalyst is treated at high temperatures, ortho-Xylene hydrogenation was performed on these two kinds of catalysts. The deactivation and selectivity values are attributed to the structural differences of the catalysts.