Xerogels texture

The very high dispersion of Pt is attributed to two parameters the xerogel texture and its surface composition. In the case of activated charcoals. 

Xerogel texture is evaluated by thermoporomotry and adsorption-desorption nitrogen isotherms (B.E.T. specific area and mesopore diameter distribution). 

Modification of silica-alumina xerogel texture by changing the mixing mode of reactants which give the silica hydrogel implies that ... 

We attempted to improve the eatalytic performanee, including stability, of the silica-immobilized Co-POM catalysts by using hydrothermally stable supports, specifically, the mesostructured silicates SBA-15 and MCF, both modified with amino groups by grafting 3-aminopropyltrietoxysilane [97], The physico-chemical properties of three representative NH2-X (X = xerogel, SBA-15 and MSF) supported Co-POM catalysts are given in Table 1. The textural properties of the initial, POM-free supports are shown for comparison. 

C. Effect of the Nature of the Organic Group on the Texture of the Xerogels... 

Quantitative determinations of the degree of polycondensation in systematic studies have demonstrated that kinetic parameters such as the temperature have little effect on the level of polycondensation in xerogels 40 and 73, and the solvent has a more important effect149,151,152. Variations of 5 to 20% are observed like those presented in Table 3. From these data and those shown in Table 4, it is evident that the degree of polycondensation and the textural properties of the xerogels are generally not related for these precursors. A variation of 5% of the degree of polycondensation is measured while the specific surface area is increased 10 fold. 

Temperature of the sol-gel processing. Only in cases of the xerogels 40, 73 and 74 has a systematic study demonstrated how much the texture is temperature-dependent. From data in Table 4, an increase in the temperature of the perforation of the gel leads to a decrease in microporosity, an increase in mesoporosity and an increase in specific surface area155. It also appears that the texture of the material is not directly correlated to the degree of polycondensation (see Table 3)151,152. The same effect was also found for the xerogel P(C6H4SiOi.5)3 (30 m2g-x at 20 °C and 340 m2g 1 at 60 °C)109. 

TABLE 4. Textural characteristics of xerogels prepared at different temperature... 

For the xerogel of 54 these peaks are always present, and their position is unchanged regardless of the texture of the solid or of the kinetic parameters (solvent, catalyst, concentration) used in the hydrolysis/polycondensation step. However, co-polymerization with various amounts of TMOS was found useful for the investigation of the structure of these materials. 

High dispersive precipitated silica submitted to an increasing pressure in a mercury porosimeter shows successively a collapse mechanism of porous texture followed by a mechanism of mercury intrusion in the part of pore network which has resisted to the collapse. Such a behavior has been previously observed on low density xerogels and on some carbon black. Both mechanisms can be clearly distinguished by a sharp variation of slope of cumulative pore volume curve versus pressure. 

Porous texture modifications of a series of silica and silica-alumina hydrogels and xerogels a thermoporometry study. 

Texture of gels has been evaluated by thermoporometry. This calorimetric method, which has been described elsewhere [5], applies to hydrogels (wet materials) as well on xerogels (dry materials). Thermoporometry is based on the analysis of solidification of pure water confined in the pores of a material. 

Figure I Comparison of the texture of silica and silica-alumina (hydrogels and xerogels) from cumulative volume curves...

It can already be noted that the texture of silica gels (hydrogel and xerogel) is characterized by a very narrow pore size distribution while silica-alumina gels have a wider pore size distribution. 

Thermoporometry applying also on xerogels can give the answers. The effects of three drying modes on gel texture have been studied and compared ... 

Table 8 depicts the effects of the preparation temperature on the texture of hydrogels and xerogels of silica and silica-alumina. As the preparation temperature is increased the pore volume and the mean pore radius of hydrogels as well as xerogels, are increased. 

Table 8 Effect of gelation temperature on the texture of hydrogels and xerogels of silica and...

As thermoporometry is suitable for hydrogel as well as for xerogel materials, it has been possible to study the effects of the operating parameters of the preparation process on the texture of silica and silica-alumina and to follow the changes carried out by each step of the process. 

The silica hydrogel network constitutes the framework of the silica-alumina gel. From a textural point of wiev, a silica-alumina xerogel seems to be an "image" of the starting silica hydrogel. Depending on the alumina content the silica-alumina gels exhibit predominantly a silica-like texture or an alumina-like texture. 

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