Silica support, increasing surface area

The former represent the weight percent of carbon atoms on the adsorbent sample measured by elemental analysis. The bonding density of a given carbon load decreases with increasing surface area of the silica support porosity multiplies the available particle surface. The specific surface area (surface area in 1 g adsorbent) is said to be inversely proportional to pore diameter (at constant specific pore volume) and obviously increases with increasing specific pore volume. 

Figure 3.6. Examples illustrating the increase of retention and resolution with increasing surface area of the silica support. Diagram courtesy of Supelco Inc.

The support material used was silica OX-50 (surface area 50 m /g) which was purchased from Degussa. Before usage the silica was boiled in bi-distilled water and dried in vacuo at 200 °C for 3 d to increase the amount of silanol groups. 

A further important aspect of carbon inertness in catalysis is its much lower coking propensity in comparison with alumina or silica supports. Coke deposition on the surface of the catalyst reduces the life of the catalyst. De Beer et al. (1984) studied this effect and found that the extent of carbon deposition on the blank supports is higher for carbons than for alumina and it increases with increasing surface area. In the absence of a metallic component the cracking appears to be related more to the accessible surface area than to any other particular surface property. However, the addition of metals to the supports causes an increase in the rates and amounts of carbon deposition, but the increase is much higher for the alumina-supported catalysts. 

Fused silica capillary columns of various internal bores and of lengths in the range 25 to 50 m are mainly employed for analytical separations. A variety of polar and non-polar column types are available including those open tubular types with simple wall coatings (WCOT), those with coatings dispersed on porous solid-supports to increase adsorbent surface area (SCOT) and porous layer open tubular (PLOT) columns. Important stationary phases include polyethylene glycol, dimethylpolysiloxane and different siloxane copolymers. Various sample introduction procedures are employed including ... 

In most cases, the SAPCs are less active than their homogeneous counterparts, but more active than their liquid biphasic catalyst analogue, a phenomenon very simply interpreted as resulting from the increase of the interphase surface area which, in the case of the SAPCs, is similar to that of the silica support. 

To increase the surface area, the resin can be supported on porous carriers, or it can be directly incorporated into silica by a sol-gel preparation technique. Both methods have been used by Botella et al. (205), who compared several composite Nafion/silica samples with varying surface areas and Nafion loadings for isobutane/2-butene alkylation at 353 K. Furthermore, supported and unsupported Nafion samples were used. As expected, the unsupported resin with its low... 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

In order to increase the contact of a catalyst with hydrogen and the compounds to be hydrogenated platinum (or other metals) is (are) precipitated on materials having large surface areas such as activated charcoal, silica gel, alumina, calcium carbonate, barium sulfate and others. Such supported catalysts are prepared by hydrogenation of solutions of the metal salts, e.g. chloroplatinic acid, in aqueous suspensions of activated charcoal or other solid substrates [28. Supported catalysts which usually contain 5, 10 or 30 weight percent of platinum are very active, and frequently pyrophoric. 

A small increase of the (d 001) basal spacing is observed for the Li containing Zr pillared clays. However, the thermal stability of these solids drastically decrease. At high temperature, the collapse of the strucutre is also supported by the decrease of the surface area which is, at 700°C, almost identical to those measured for the montmorillonite. Different hypothesis may be proposed to explain the increase of the interlayer distance at low temperature (i) a better polymerization of the intercalated complex (ii) a modification of the distribution of the pillars (iii) a lower interaction between the pillar and the silica layer. The first hypothesis may easily be eliminated since the small variation of the height of the pillars (less than 1 A) cannot be explained by structural changes of the... 

A variety of industrial catalytic processes employ small metal-particle catalysts on porous inorganic supports. The particle sizes are increasingly in the nanometre size range which gives rise to nanocatalysts. As described in chapter 1, commonly used supports are ceramic oxides, like alumina and silica, or carbon. Metal (or metallic) catalysts in catalytic technologies contain a high dispersion of nanoscopic metal particles on ceramic oxide or carbon supports. This is to maximize the surface area with a minimum amount of metal for catalytic reactions. It is desirable to have all of the metal exposed to reactants. 

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