To date, a complete assessment of the so-called pore-size distribution by adsorption methods has, in general, not been accomplished. This is because the diverse capillary phenomena involved in such a description of the porous structure usually undergo cooperative and/or percolative interactions which hinder complete characterization. The object of the present work was to suggest a general method for the analysis of texture and to provide specific examples which identify and describe some of the problems and possibilities of the textural characterization of porous substrata from vapour adsorption studies. A series of alumina products of very assorted morphologies were synthesized using a modified Yoldas sol-gel method, and used to demonstrate important morphological and mechanical aspects of the pore structure from the observation and analysis of their nitrogen adsorption hysteresis loops at 76 K.
Raney-type catalysts are widely applied in a variety of commercial processes in the chemical industries. Conventionally, Raney nickel catalysts are produced by leaching aluminum from cast Al-Ni alloys; they consist of porous nickel particles having small amounts (3-5 wt. %) of residual aluminum and surface areas in the range of 50-130 m 2 /g [1]. In recent years, reports have been made regarding the production of Raney catalysts by leaching of precursor alloys obtained by mechanical alloying (MA) [2][3][4]. In these studies, the microstructural characterization has been quite limited in detail. The results of our own work with Raney nickel and nickel-iron catalysts have clearly shown that, compared to conventional Raney catalysts, those derived from precursor alloys prepared by MA possess important advantages regarding simplicity and reproducibility of production, as well as catalytic performance. In order to better understand these findings, we have now conducted a comparative microstructural characterization of conventional and MA Raney catalysts.Catalyst preparation has been reported elsewhere [3,4]. Materials characterized in the present study included the as-milled MA alloys before and after leaching. For comparison, it was included a commercial Raney nickel catalyst. All materials were characterized by XRD, SEM, TEM and EDS.Before leaching, the MA powders consisted mainly of nodular particles with sizes mostly in the range 0.5-2 µm (Fig. 1a). The individual particles contained crystallites with sizes mainly in the 5 -10 nm range (not shown). SAD patterns obtained from individual particles gave clear evidence for the existence of the NiAl intermetallic phase, in agreement with the results of XRD.
INDICE DE TABLAS Tablas Descripción Páginas 2.1 Principales modelos de los materiales porosos según 19 Karnaukov. 3.1 Condiciones de síntesis de alúminas con diferentes propiedades 66 texturales. 4.1 Condiciones de síntesis de dióxido de zirconio en matriz de sílice 88 con diferentes propiedades texturales. 4.2 Mediciones de la fuerza ácida del sistema dióxido de zirconio en 90 matriz de sílice. 4.3 Efecto del contenido de Zr02 sobre la actividad catalítica de 94 Pt/(SO4)-Zr0z-Si02. 5.1 Condiciones de síntesis de óxidos mixtos de níquel-sílice con 129 diferentes propiedades texturales. 5.2 Características de los catalizadores preparados vía sol-gel. 132 5.3 Resultados obtenidos con difracción central de rayos X. 135 Patrones de difracción de rayos X de las muestras de níquel sintetizadas. Cristalinidad de las fase presente a 400 *C. 5.3 5.4 5.5 Determinación del tamafío de cristal con DRX de muestras de NiO-302, obtenidos mediante SOL-GEL. Espectros FDR de los prototipos níquel sílice. 137 Microfotografía de transmisión de Ni-Si02 composición 5% NiO-V I J Fs(R) y FB(R) Son las funciones de densidad de probabilidad de distribución de tamaños de sitios y enlaces, respectivamente, establecidas de manera normalizada sobre una base de número de elementos. Probabilidad de encontrar un sitio de tamaño R. Probabilidad de encontrar un enlace de tamaño R. Presión relativa. Constante de los gases. Temperatura absoluta. Tensión superficial. Volumen molar del adsorbato. Curva límite ascendente. Curva límite descendente. Volumen de un sitio de tamaño R. Volumen de un enlace de tamaño R. Areas correspondientes a los sitios y enlaces. Números totales de sitios y enlaces. Volúmenes totales de poros y del sólido.
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