V.H. Lara

A catalytic application of Cu2O and CuO films deposited over fiberglass

Abstract With the purpose of utilizing copper oxides over fiberglass as a novel catalyst material in oxidation reactions, we have used the chemical vapor deposition of 2,4-pentanedionate copper(II) as a precursor and fiberglass as substrate. The deposition of copper oxides was achieved in a horizontal-flow reactor using O2 as carrier-reactant gas at several deposition temperatures. Copper species deposited on fiberglass were characterized by X-ray diffraction (XRD), UV–visible spectrophotometry (UV–VIs) and scanning electronic microscopy (SEM) techniques. The fiberglass coated with copper oxides showed a catalytic activity for the oxidation of ethanol into a stream of air at different temperatures in a pulse microreactor.

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.

Nitrogen-Sorption Characterization of the Microporous Structure of Clinoptilolite-Type Zeolites

The micropore-filling characteristics of a series of natural and modified microporous clinoptilolite-type zeolites with N2 at 76 K are measured and analysed. The adsorption behaviour of these substrata is examined in the range of relative pressures between 10−5–1. Several methods such as: Sings αs-plots, de Boers t-plots, Lee and Newnhams direct comparison plots, Dubinins classical methods and a difference isotherm procedure proposed here, are used to assess the microporosity of the samples. Natural samples are used as reference materials to perform these sorption analyses of the modified samples. The effect of narrow micropore constrictions on the adsorption behaviour of clinoptilolites is explored experimentally. The occurrence of a low-pressure hysteresis loop along the sorption isotherm of a modified sample is frequently found and may be due to the strong adsorption of adsorbate molecules at the entrance of necked micropores that interfere with the diffusion of adsorbate molecules inside the porous structure of these zeolites.

Microwave Irradiation Effect on Hydrotalcite Synthesis

New synthesis method of hydrotalcites using microwave irradiation is presented, which avoids the autoclave high temperature treatment as well as the long crystallization time. The crystallite size of the obtained solids is smaller than in the conventionally prepared hydrotalcites. The highest irradiation time provided the lowest aluminium incorporation to the hydrotalcite network. Diminishing microwave irradiation time produces purer hydrotalcites. If the gel is irradiated only for two minutes a well crystallized hydrotalcite is obtained.

Effect of microwave irradiation time on the synthesis of zirconia-pillared clays

In this work, we present the synthesis, at different zirconia/clay ratios, of zirconia-pillared montmorillonite in presence of microwave irradiation. The effect of irradiation time (5, 10, 20 and 30 min) on the structure and properties of the solids was also studied. The samples were characterized by X-ray diffraction, nitrogen adsorption (BET surface area) and thermo-programmed desorption of ammonia. It is shown that the irradiation time is critical between 5 and 10 min. For times longer than 10 min, no effect on the pillared clay features is observed.

Co(II)–Co(III) hydrotalcite-like compounds

Abstract The synthesis of monometallic hydrotalcite-like compounds (MHLC) type Co(II)–Co(III) materials was achieved by precipitation of a solution of cobalt nitrate(II) followed by microwave irradiation during the hydrothermal treatment step. The effects of irradiation time, precipitant agent and atmosphere were studied in order to establish the preparation conditions and the stability of these materials. As expected, these materials required careful preparation conditions, mainly a very slow addition of the precursors. NH 4 OH had to be used as precipitant agent in an air atmosphere. Short microwave irradiation times during the hydrothermal treatment provided a better crystallization of MHLC. The low stability of these compounds (∼200°C) was explained by the presence of several Co-complexes between the layers of the laminar structure. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal analysis and thermogravimetric analysis (DTA/TGA), and thermoprogrammed oxidation (TPO) were used as characterization techniques.

Thermokinetic study of the rehydration process of a calcined MgAl-layered double hydroxide.

The rehydration process of a calcined MgAl-layered double hydroxide (LDH) with a Mg/Al molar ratio of 3 was systematically analyzed at different temperatures and relative humidity. Qualitative and quantitative experiments were done. In the first set of samples, the temperature or the relative humidity was varied, fixing the second variable. Both adsorption and absorption phenomena were present; absorption process was associated to the LDH regeneration. Of course, in all cases the LDH regeneration was confirmed by other techniques such as TGA, solid state NMR, and SAXS. In the second set of experiments, a kinetic analysis was performed, the results allowed to obtain different activation enthalpies for the LDH regeneration as a function of the relative humidity. The activation enthalpies varied from 137.6 to 83.3 kJ/mol as a function of the relative humidity (50 and 80%, respectively). All these experiments showed that LDH regeneration is highly dependent on the temperature and relative humidity.

Hydrolysis catalyst effect on sol–gel silica structure

Abstract Silica samples were prepared by the sol–gel process varying the hydrolysis catalyst. The samples were characterized by X-ray diffraction and the radial distribution function was determined (R.D.F.). Fourier transform infrared spectroscopy and thermal analysis were also used. It was found that OH-retention capacity as well as order in the material may be modified with hydrolysis catalysts. When the hydrolysis catalyst was NH 4 OH the sample did not dehydroxylate even at 1000°C.

Detrital Mg(OH)2 and Al(OH)3 in Microwaved Hydrotalcites

Microwave irradiation power (90, 180, 270 or 360 W) determines the composition and the structure of the compounds obtained when pH is increased, decreased or maintained constant during preparation. Although in all cases the X-ray diffraction patterns show that the obtained compounds are hydrotalcite-like, the surface areas (40 up to 240 m2/g) as well as other microcrystalline compounds have to be inferred, such as Al(OH)3 or Mg(OH)2.

Structural and textural evolution during folding of layers of layered double hydroxides.

Layers of a layered double hydroxide, containing aluminum 4-fold coordinated, were partially folded in order to obtain a fibrous hydrotalcite-like compound. The hydrotalcite layers, in the presence of an anionic surfactant (sodium dodecyl sulfate) after hydrothermal treatment for 2 weeks, acquire a mesoporous-like arrangement. The transformation was monitored by techniques sensitive to structural and textural properties. Results suggest that brucite-like layers can be joined throughout unsaturated coordinated aluminum, that is, tetrahedral aluminum which links through hydrogen bonds to form aluminum octahedrally coordinated. The fractal dimension parameter was very sensitive to evolution from layered to fibrous hydrotalcites.