L.C. Costa

The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity

In this article, we considered all aspects of acidity (nature of acid sites, strength, density, etc.) in solid catalysts and in zeolites in particular. After reminding the definition of acidity in liquid and solid acids, we emphasized acidity characterization by the most used physical techniques, such as Hammetts indicator titration, microcalorimetry of adsorbed probe molecules (ammonia, pyridine or other amines for acidity characterization and CO2 or SO2 for basicity characterization), ammonia or any amine thermodesorption, IR spectroscopy of hydroxyl groups and of several probe molecules adsorbed (ammonia, pyridine, piperidine, amines, CO, H2, etc.), MAS-NMR of 27Al, 29Si, 1H elements and of 1H, 13C, 31P, etc. of adsorbed probe molecules, and model catalytic reactions. Modeling the way the acid features of zeolites influence the catalytic activity of these catalysts toward acid-catalyzed reactions (relation between ammonia desorption activation energy values and catalytic activities, reaction mechanism, and kinetics) completes the general analysis of acidity and zeolite chemistry.

Electric modulus-based analysis of the dielectric relaxation in carbon black loaded polymer composites

A thorough investigation of the ac electrical properties of carbon black (CB) mixed into ethylene butylacrylate copolymer has been conducted using ac impedance spectroscopy in the frequency range 10 Hz–100 kHz and over the temperature range of 150 (Tg−48 K)–319 (Tg+121 K) K. For this investigation, a series of eight samples were prepared with various filler contents above the percolation threshold. Using the electric modulus formalism it has been found that the Cole–Cole equation of dielectric relaxation expressed in the electric modulus form is capable of quantitatively describing the experimental data from which we extract the relaxation time and a parameter α which gauges the broadening of the loss spectrum. The small values of α ranging from 0.06 to 0.10 suggest a behavior close to the state of a single relaxation time. Furthermore, the relaxation time as a function of temperature is characterized by an Arrhenius behavior. While the effective activation energy is on the order of 60 meV and is insensit...

Enhanced dielectric response of GeO2-doped CaCu3Ti4O12 ceramics

CaCu3Ti4O12 ceramic samples were prepared by solid state conventional route using stoichiometric amounts of CuO, TiO2, and CaCO3. Afterward the material was doped with GeO2 with concentrations up to 6% by weight and sintered at 1050 °C for 12 h. The influence of doping on the microstructure, vibrational modes, and dielectric properties of the material was investigated by x-ray diffraction, scanning electron microscopy coupled with an energy dispersive spectrometer, and infrared and dielectric measurements between 100 Hz and 30 MHz. The materials presented huge dielectric response, which increases with doping level relative to undoped CaCu3Ti4O12. The main effect of doping on the microstructure is the segregation of Cu-rich phase in the ceramic grain boundaries. Cole–Cole modeling correlates well the effects of this segregation with the relaxation parameters obtained. The intrinsic phonon contributions for the dielectric response were obtained and discussed together with the structural evolution of the system.

Facile synthesis of hydrogenated reduced graphene oxide via hydrogen spillover mechanism

Here we demonstrate a single step approach for the facile reduction of graphene oxide (GO) to hydrogenated reduced graphene oxide (HRGO) under ambient conditions.

Structural investigation of the silica-titania gel/glass transition

Vitreous silica containing titania has received considerable attention due to its anomalous thermal expansion behaviour, together with high optical quality. The effect and the role of TiO2 in the properties of glass are directly correlated with the structural position of titanium; thus several attempts have been made to elucidate the structural aspects of gels and glasses containing titania. Several SiO2-TiO2 gels containing up to 30 mol% TiO2 prepared using (i) normal drying; (ii) supercritical drying in alcohol; (iii) supercritical drying in CO2 were studied in this work. The aim of this work is to establish the titanium coordination during thermal treatments during the gel/glass system evolution. X-ray diffraction (XRD) and dilatometric analysis have been used to detect the presence of a crystal phase of TiO2 (anatase) in gel/glass samples. The crystal size of anatase was evaluated using the Scherrer relation from XRD patterns. Infrared and diffuse reflectance spectroscopy in the UV-visible region have been used to elucidate the structural aspects and the electronic transitions due to different titanium coordination. These results indicate that drying conditions strongly influence the silicon-oxygen-titanium structure.

A green-emitting CdSe/Poly(butyl acrylate) nanocomposite

CdSe/poly(butyl acrylate) nanocomposites were synthesized by in situ miniemulsion polymerization. The hybrid nanomaterial is very stable and presents a bright green photoluminescence at 2.29 eV under ultraviolet excitation. With the excitation conditions used the intensity of the emission band keeps nearly constant from 7 K to room temperature. The morphological, structural and room temperature electrical properties of the CdSe/poly(butyl acrylate) nanocomposite have been investigated.

Light emitting porous silicon diode based on a silicon/porous silicon heterojunction

A new structure is proposed to improve the external quantum efficiency of porous silicon (PS) light emitting diodes (LED). It is based on a heterojunction between n-type doped silicon and PS. The heterojunction is formed due to the doping selectivity of the etching process used to form PS. The improvement of the proposed LED structure with respect to usual metal/PS LED is demonstrated. This is thought to be due to a different injection mechanism for which carriers are injected directly into conduction band states. Anodic oxidation experiments show further improvements in the LED efficiency.

Free and/or bound water by dielectric measurements

Abstract In the past few years, the research on new methods (FTIR, NMR, microwaves) to measure the quantity and quality of water has been largely improved, particularly in the agricultural industry. Numerous studies have been made in order to establish a relationship (BET, GAB, …) between parameters such as temperature, the water content in the matrix and the partial pressure of water vapour. The dielectric spectroscopy in the microwave domain shows a strong discrimination between bound and free water. The measurements in a resonant cavity allow calculation of the complex permittivity. This methodology has been used to quantify the grade of a seeds viability, and by extension, to discriminate between living seeds, containing mostly bound water, and dead seeds, containing particularly free water. This was explored with the society CLAUSE, in tomato, pimento and melon seeds. In a resonant cavity, the dielectric response of each seed is dependent on the weight, the heterogeneity of the shape (depolarisation factor of the electric field), and the anisotropy of density distribution of water in the matrix. Consequently, in order to take into account these parameters, the rotation of the seed in the electric field gives the total dielectric response or the dielectric signature of each seed. In this first approach, using only four perpendicular positions of the seed in the cavity, the mathematical envelope of the dielectric response is usually greater when the seed is dead.

Dielectric behaviour of Nd ions in the lead borate glass

Abstract Accurate measurements of the relaxation in the time domain show that the low frequency relaxation processes in glasses are well described by the Williams-Watts (WW) relation. Measurement of the complex permittivity in the frequency domain have been oftein claimed, on the other hand, to satisfy the Cole-Davidson (CD) relation, e ∗ =e ∞ +[ (e 0 −e ∞ ) (1+jωρ) a ], 0 . In order to check with what accuracy the systems whose time domain behaviour is given by WW relation also satisfy the CD equation in the frequency measurements of the glasses in the frequency region 1 Hz-10 GHz are taken using different techniques. Absorption was observed in two different frequency regions and they are well described by the two relations. The temperature dependence of relaxation time parameters was also studied in order to characterize the physics of the relaxation process. The dependence of the parameters on the concentration of Nd sheds some light on the state of aggregation of the Nd ions in glass.

Sol-gel formation of silica-zirconia glasses

Abstract Bulk samples of gels with composition (100 − x)SiO2 − xZrO2, x = 10 or 30, were prepared using TEOS and Zr(OC3H7)4, complexed 1:1 with acetylacetone, as precursors. Samples of both compositions were dried under hypercritical conditions using ethyl alcohol or liquid CO2 as solvent, as well as slowly dried in air as xerogels. The aim of this work was to verify whether different drying methods could lead to different crystallization behaviours during the gel-to-glass transition. Dried gels were characterized by X-ray diffraction, TGA-DTA and dilatometry.