Surface areas of carriers

While the specific surface area of carriers is often determined—as we have seen above—by using physical adsorption, the active phase of the catalysts (metal, oxide, sulphur) can be studied by selective chemisorption (with no support interaction) of an adsorbate under conditions of pressure and temperature permitting the formation of a single layer on the surface of the metal. During chemical adsorption, there is a chemical reaction between the gas molecule and the active phase, which is represented by the transfer or sharing of electrons. 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

The platinum concentrations in the platinized carbon blacks are reported to be between 10 and 40% (by mass), sometimes even higher. At low concentrations the specific surface area of the platinum on carbon is as high as lOOm /g, whereas unsupported disperse platinum has surface areas not higher than 10 to 15m /g. However, at low platinum concentrations, thicker catalyst layers must be applied, which makes reactant transport to reaction sites more difficult. The degree of dispersion and catalytic activity of the platinum depend not only on its concentration on the carrier but also on the chemical or electrochemical method used to deposit it. 

Carrier-mediated transport is linear with mucosal solute concentration until this concentration exceeds the number of available carriers. At this point the maximal solute flux (7max) is independent of further increases in mucosal solute concentration. In the linear range of solute flux versus mucosal concentration (C), the proportionality constant is the ratio of / to the solute-carrier affinity constant (Km). This description of Michaelis-Menten kinetics is directly analogous to time changes in mass per unit volume (velocity of concentration change) found in enzyme kinetics, while here the appropriate description is the time change in solute mass per unit surface area of membrane supporting the carrier. 

Gas chromatography (GC) employs a gaseous mobile phase, known as the carrier gas. In gas-liquid chromatography (GLC) the stationary phase is a liquid held on the surface and in the pores of a nominally inert solid support. By far the most commonly used support is diatomaceous silica, in the form of pink crushed firebrick, white diatomite filter aids or proprietary variants. Typical surface areas of 0.5-4 m2/g give an equivalent film thickness of 0.05-1 pm for normal liquid/support loadings of 5-50 per cent by mass. 

If the flow of the carrier gas (e.g., He) is given by Fg (cm3 s 1) and An is the change in the trace gas concentration due to uptake by the droplets, then the number of gas molecules taken up per second is just FgAn. The number of gas-droplet collisions per second per unit area is given (Eq. PP) as J = NgudV/4, where N is the number of gas molecules per unit volume and wav is the mean molecular (thermal) speed. If Ad is the surface area of one droplet and there are N droplets to which the gas is exposed, then the total available surface area is (N Ad), the total number of gas-droplet collisions is J = (N Ad)NgudV/4, and the measured mass accommodation coefficient becomes... 

There may be competing factors between emulsion size and extractable surface oil that produce these results. While the finer emulsions have less extractable surface oil which should improve shelf-stability, the total surface area of the oil droplets in these powders is greater (Table V). The lower amount of surface oil provides less oil that is openly exposed to oxidation but the greater surface area of the droplets in the carrier matrix provides greater possibility for oxidation once oxygen has permeated the spray dried particles. 

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