Specific outer surface area

CTAB surface area m2/g ASTM D 3765 ISO 6810 adsorption of CTAB (cetyl trimethyl ammonium bromide) from aqueous solution as a measure of the specific outer surface area... 

The simplest kind of a fixed catalyst bed is a random packing of catalyst particles in a tube. Different particle shapes are in use like spheres, cylinders, rings, flat disc pellets or crushed material of a certain sieve fraction. Mean particle diameters range from 2 to 10 mm, the minimum diameter is limited primarily by pressure drop considerations, the maximum diameter by the specific outer surface area for mass and heat transfer. 

The mass transfer time is given by the mass transfer coefficient in the fluid, ka/ and the specific outer surface area of the catalyst, a. For microreactors with catalyst-covered walls, a can be obtained by dividing the covered reactor surface area by the reactor volume... 

The characteristic time for the external mass transfer between the bulk fluid and the surface (t ) depends on the mass transfer coefficient in the fluid and the outer surface-to-particle volume ratio commonly called specific outer surface area a (Eq. (11.3)). 

An important characteristic of a fixed bed of particles is the specific outer surface area of the particles. When the particles all have the same diameter, the specific surface area a can be related to the void fraction e of the bed and the solids fraction e by the following equation ... 

When there is an interaction as indicated above, the maximum reaction rate can be increased by the enhancement factor that can be 1, when the specific outer surface area of the catalyst is large enough. It has been shown that with commercial catalysts of the type noble metal on carbon, this enhancement factor can easily be > 3 (Vinke et al. 1992, Beenackers and Van Swaaij, 1993). 

Sorption Analysis. Specific surface areas and porosity can be calculated from the adsorption isotherm of nitrogen at — 196 °C. The method of Brunauer, Emmett, and Teller [4.29] is generally accepted for the evaluation of specific surface areas (BET surface area in square meters per gram). The two-parameter equation is applicable to carbon black. The BET surface area comprises the outer surface area as well as the surface area of the pores. 

The objective is to produce a plasticized material that is easily shaped in a die plate and has a specific porosity with a defined pore size and a large inner and outer surface area for a good catalytic activity. Therefore, shear is applied to the ceramic to move the particles against each other so that all hollow spaces between the solid particles can be filled with the liquid binder (see Fig. 13) and the so called kneading dough is formed p riSO]. The dispersed binder provides the extruded profile with the strength... 

Adsorption from the gas phase is commonly applied in determining the specific surface area of finely dispersed materials. For that purpose, assumptions have to be made concerning the dimensions of the gas molecules and the structure of the adsorbed layer under saturation conditions (fully packed monolayer, multilayer, etc.). Small gas molecules may enter pores and capillaries in porous materials. Hence, by comparing the surface area determined by gas adsorption with the outer surface area obtained from, for example, electron microscopy, the porosity of the material can be estimated. Moreover, by using different types of gas having different molecular dimensions, an impression of the pore size distribution may be obtained. 

Amount of cetytrimethylammonium bromide adsorbed from aqueous solution as measure of specific nonporous (outer) surface area... 

Many cells have an asymmetric structure because of the necessity for function (Drubin and Nelson, 1996). For example, (the outer surface of) the plasma membrane of epithelial cells is fenced by a tight junction so that the lipids are separated between the apical part and the basolateral part (Fig. 9) (Eaton and Simons, 1995). Therefore, some molecular mechanisms must exist to sort the plasma membrane proteins into these two parts. Some signals related to the secretory/endocytic pathways have been found important (Matter and Mellman, 1994). Their details are not described here because the area is too specific for predictive purposes. 

Figure 5.14 Hydrogen molecule on the surface of a SWNT (5,5). The van der Waals interaction between H2 and the curved surface of the nanotube is weaker on the outer surface and stronger on the inner surface. Furthermore, the specific surface area of the adsorbate layer (Rads) is larger than that of the nanotube (Rnt)-...

Specific Surface Area. The specific surface area of industrial carbon blacks varies widely. While coarse thermal blacks have specific surface areas as small as 8 m2/g, the finest pigment grades can have specific surface areas as large as 1000 m2/g. The specific surface areas of carbon blacks used as reinforcing fillers in tire treads lie between 80 and 150 m2/g. In general, carbon blacks with specific surface areas >150 m2/g are porous with pore diameters of less than 1.0 nm. The area within the pores of high-surface-area carbon blacks can exceed the outer (geometrical) surface area of the particles. 

To determine the mean primary particle size and particle size distribution, the diameters of 3000-5000 particles are measured on electron micrographs of known magnification. Spherical shape is anticipated for calculations. However, since the primary particles generally build up larger aggregates, the results may be somewhat uncertain. The specific electron microscopic surface area can be calculated from the primary particle size distribution. This value refers only to the outer (geometrical) surface of the particles. For porous carbon blacks the electron microscopic surface area is lower than the specific surface area according to BET (see below). 

There are various kinds of polytetrafluoroethylene. One is granular polymer consisting of spongy, white particles having a median size of about 600/l The specific surface of this polymer is on the order of 2 m2/g (determined by nitrogen adsorption and calculations by the method of Brunauer, Emmett, and Teller). Since this specific surface area is about 1700 times the observed outer surface of the particles, these measurements confirm the porous, spongelike structure that can be seen in the photomicrograph of a cross section of several particles in Fig. la. 

Stabilizers can be introduced into the pellets or the washcoats with the intention of slowing down the thermally induced decrease in the surface area of the porous structure itself, or of the active component. Both, the active materials and the stabilizers, are put sometimes only on the outer layers of the pellets or monoliths, while, in other cases they penetrate the porous structures completely. Such preferential distributions have very specific aims, the utilization of the active materials and their protection from poisoning being the most important ones. There exists a vast body of patent literature on such designs. 

When considering particle size and its distribution, typically the particle size D3 2 (the average diameter if all particles were to have the same outer surface) is more important than D4 3 (the average diameter if all particles were to have the same volume), which in turn is more important than a D50 average diameter, which then again is more important than the specific surface area (SA) (m2 g"1). For a certain threshold particle size, the results obtained can be much better than for larger particles. 

---