Síntesis y caracterización de xerogeles de sílice obtenidos por la ruta de los atranos

En este trabajo se investigó la síntesis de xerogeles de sílice por la vía de los atranos, y se evaluó la influencia de la concentración del agente iniciador (HCl) y la presencia o no del surfactante (CTAB), sobre el tiempo de gelificación y las propiedades texturales de los materiales obtenidos. Las caracterizaciones se realizaron mediante: isotermas de adsorción-desorción de nitrógeno, microscopía electrónica de barrido y calorimetría diferencial de barrido. Los tiempos de gelificación aumentaron en la medida que se disminuyó la concentración del HCl y, en general, los xerogeles preparados presentaron una buena rigidez cuando estos se dejaron a tiempos mayores de 20 horas. La distribución de tamaño de poro (determinada mediante la técnica BJH) para los xerogeles calcinados preparados sin surfactante presentaron un sistema de poro bien definido de 16,4 nm en promedio, mientras los xerogeles calcinados preparados con surfactante no presentaron una distribución de tamaño de poro bien definida, ambos casos mostraron áreas superficiales de alrededor de 580 m2/g. Por calorimetría diferencial de barrido se observaron dos picos para la muestra de xerogel sin surfactante, uno alrededor de 80 °C debido a la evaporación del agua y el otro a 265 °C atribuido a la descomposición de la materia orgánica presente en el gel; para la muestra de xerogel con surfactante se observó un pico bien definido a 130 °C, atribuido a la pérdida del agua. Por microscopía electrónica de barrido, en los xerogeles calcinados se observaron poros con tamaños alrededor de los 15 nm. Palabra clave: Xerogel, atrano, surfactante, sílice, gelificación. Abstract In this work, the synthesis of silica xerogels by the atrane way was investigated, evaluating: concentration influence of the initiating agent (HCl) and the presence or not of the surfactant (CTAB), over gelation time, and the textural properties of the obtained materials. Characterizations were carried out by nitrogen adsorption-desorption isotherms, scanning electron microscopy, and differential scanning calorimetry. Gelation times increased as the HCl concentration decreased, and, in general, xerogels prepared presented good rigidity when they were aging for times greater than 20 hours. Pore size distribution (determined by the BJH technique) for the calcined xerogels prepared without surfactant presented a well-defined pore system of 16.4 nm on average, while the calcined xerogels prepared with surfactant did not present a well-defined pore size distribution, both cases showed surface areas of around 580 m2/g. In differential scanning calorimetry, two peaks were observed for the xerogel sample without surfactant, one around 80 °C due to water evaporation, and the other one at 265 °C attributed to the decomposition of organic matter present in the gel; for the surfactant xerogel sample, a well-defined peak was observed at 130 °C, attributed to the loss of water. By scanning electron microscopy, pores with sizes around 15 nm in calcined xerogels were observed. Keywords: Xerogel, atrane, surfactant, silica, gelation.

The Influence of the Activation Temperature on the Structural Properties of the Activated Carbon Xerogels and Their Electrochemical Performance

The effect of activation temperature on the structural properties and the electrochemical performance of KOH-activated carbon xerogel was investigated in range of 700 to 1000°C. At a high temperature (1000°C), the chemical activation regenerated a more crystalline network structure of activated carbon xerogels, which was observed by Raman, XRD, and TEM images. Additionally, SEM images, BET, BJH, and t-plot were used to study the structural properties of carbon xerogels. The carbon xerogel sample activated at 900°C was found with the most appropriate structure, which has the high micropore area and a more-balanced porosity between the micropores and mesopores, for using as an electrode material. The highest obtained specific capacitance value was 270 Fg−1 in 6 M KOH electrolyte at scan rate of 5 mVs−1 from the cyclic voltammetry.

New Synthesis Method to Obtain Pd Nano-Crystals

We report a new synthesis method to obtain palladium nano-crystals by sol-gel polymerized with acrylamide. From thermogravimetric analysis (TGA) studies, we found PdO and Pd compounds in the xerogel sample, at 550 °C, and over 900 °C we detected only metallic Pd. These results were corroborated by powder X-Ray Diffraction (XRD), High Resolution Scanning Electron Microscopy (HRSEM), and Transmission Electron Microscopy (TEM). XRD studies exhibit the lines from the tetragonal structure (PDF 41-1107) of PdO compound and from the cubic structure (PDF 46-1043) of Pd metallic. HRSEM micrographs show morphologies from the sample very sensitive to heat treatment. Finally, TEM images show crystals of ~8 nm in diameter.