Different Surface Areas

The electrochemical technique for determining the s6h values as a measure for the maximum available surface area of platinum in porous electrode structures was already discussed in section 5 of chapter VI. It is evident from the introductory remarks to this chapter, that the electrodes have to be in the flooded state for this type of measurement. Determinations of the maximum available surface of the electrocatalyst from the adsorbed amount of hydrogen or carbon monoxide in the gas phase yield values that are too large if many isolated clusters of electrocatalyst are present. 

The quantity Ag found as specific surface area by the BET technique [21] in m /g is a measure of the maximum available surface of porous metal structures if metal and electrocatalyst are the same. The situation becomes more complex when the porous structure consists of a binder and an electrocatalyst. It is considered likely that the parameter Ag is proportional to the maximum available surface under the following conditions  

Conditions a and b are not met by Teflon-bonded platinum black electrodes, so that a correlation between Ag and the performance appears questionable. Schwabe [10] suggested that the number of active sites for the O2 reduction on porous carbon electrodes is independent of Ab, as in the case of numerous catalytic reactions. 

The fraction of the maximum available surface of electrocatalyst that participates in electrochemical reactions under conditions of gas diffusion will depend upon numerous parameters like conductivity of electrolyte, temperature, gas pressure, pore size distribution, hydro-phobicity, rate-determining step of the electrochemical reaction, electrode potential, etc. An order of magnitude may be obtained [12] for this fraction by the ratio of the values of the double layer capacity under diffusion conditions and flooded conditions. 

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Fumed sihca, a highly reinforcing filler, is usually added in amounts ranging from 6 to 20%. Sihca is most often used when a high strength sealant is desired. Several sihcas having different surface areas are available and surface treatment with silanes may be used as well. 

Pto,8Reo,2)ML/Pd/C. Although comparing the activity of the Pt/C electrocatalyst with an average particle size of 3.1 nm with that of PtMu/Pd/C electrocatalyst of 9 nm [Zhang et al., 2004] does not provide an adequate activity assessment because of their different surface areas, this does not adversely affect the main conclusions derived from experiments on single-crystal surfaces. 

Different surface areas may possess different adsorptive and catalytic properties. This may be because the position of the Fermi level relative to the energy bands varies from one part of the surface to another, this occurring because of the irregular distribution of impurities on the semiconductor surface (Sec. IX,A). 

To completely optimize the residue catalyst, other parameters than the different surface areas also must be optimized. For a catalyst cracking North Sea atmospheric residues, the pore size distribution also must be optimized. Pores in the mesopore range that is, pores with diameters between 50 and 500 Angstrom, are most important for precracking of resid molecules [21,23]. The possibility to make nickel and vanadium inactive is also important to optimize. 

When sodium resinate was incorporated into pastes made from two cements having significantly differing surface areas, the results given in Table 3.3 were obtained, indicating that the sodium resinate produces a mueh greater increase in viscosity in paste made from the fine cement than in pastes made from the coarse eement. 

Figure 9.61 shows some typical plots to test Eq. (VV) for some SOC in the form of higher molecular weight alkanes and some PAH (see Chapter 10) at various relative humidities (Storey et al., 1995). In this case, the gas-particle partitioning coefficient Kp has been normalized to take into account different surface areas by using a surface area normalized coefficient, Kp s, defined as... 

Several factors influence TGA data. Sample size and shape affect the rate and efficiency of decomposition. Powdered versus solid bulk samples will have different decomposition profiles due to the differing surface areas from which exiting decomposition products can leave the sample and be registered as mass losses. Similarly, the packing of the sample in the pan must be even and reproducible from run to run. Loosely distributed particles will heat more evenly and evolve volatilized products more evenly than mounded or densely packed samples. This can be especially important when looking at determinations of residual solvents, moisture or diffusion controlled losses such as plasticizer in the samples. 

Adkins and Nissen, ] for example, found that alumina prepared in different ways exhibited very varying catalytic activity towards the decomposition of formic acid. This could hardly be due simply to the different surface areas of the several preparations, since the variation was not confined to the total speed of reaction but affected also the relative speeds of the two alternative decompositions which formic acid undergoes. Moreover, the apparent heats of activation varied from one kind of alumina to another. J... 

A sodium aluminate solution (2.00 moles Al203/liter, 3.40 moles Na20/ liter, density 1.36 grams/ml) produced from hydrargillite was used as the alumina source in each series of tests. Silica was used as sodium silicate solution (5.98 moles Si02/liter, 1.77 moles Na20/liter, density 1.36 grams/ ml) and as solid amorphous silica with different surface areas (Table I). 

Fig. 58. Rates of catalytic oxidation of H2 and of CO in the presence of catalysts having different surface areas, (a) H3PM012O40 at 573 K, (b) Na2HPMO 20.to at 623 K. ( ) H2-02 (A) C0-02 (From Ref. 262.)...

Given the same bulk density and binder/filler ratio, an increase in the space factor of the polystyrene granules from 52 to 62 vol % decreased the macrosphere density from 270 to 220 kg/m3 63). In order to obtain macrospheres with the same density, their volumes should be the same, even though they may have different surface areas. 

The resistance of the electrolyte, Ra, represents the ohmic resistance of the complete column of electrolyte between both electrodes. Note that in this way of presentation, the Faradaic impedance cannot necessarily be correlated with an occurring process, therefore it is represented by a general symbol ZF. It can be seen in this case that the total impedance consists of contributions from the working and the counter/reference electrodes, and knowledge of the behaviour of only the working electrode needs manipulation. A simple possibility is to use a CE/RE electrode with much larger surface than the WE. However, in the work presented in this book, this manipulation is not done, because the system used here will be used in applications as a set of two identical electrodes. In addition, one of the parameters that we are interested in is the electrolyte resistance, a parameter that is suppressed when using electrodes of different surface areas. 

Different surface area of contact, i.e., changes in surface area to volume ratio... 

Sufficient DO data were not obtained from basalt-synthetic Grande Ronde groundwater experiments to allow determination of a definitive rate law. A first order kinetic model with respect to DO concentration was assumed. Rate control by diffusion kinetics and by surface-reaction mechanisms result in solution composition cnanges with different surface area and time dependencies (32,39). Therefore, by varying reactant surface area, determination of the proper functional form of the integrated rate equation for basalt-water redox reactions is possible. 

In order to describe what happens in the molecular organization at smaller areas and understand the process undergone by water molecules and counterions under these conditions, it was also developed molecular modeling of a P4VPCi4 mono-layer with full quaternization. Two different surface areas were taken into account. One would correspond to the limiting area A0 (34 A2) and another to a larger area (40 A2) than the first one. When the compression is beginning to reach 40 A2, the lateral chains could be considered almost vertical at the air-water interface. (See Fig. 3.15). The situation shown in Fig. 3.15 seems to favor hydrophobic interaction between them. 

Similar studies on foam morphology were reported by Williams et al.12,13 for polyimide foams with different densities or surface area also two different chemical formulations were used. Comparing foams with the same chemical composition, it was shown that no consistent correlation could be found between PHRR and foam density or open cell content while greater correlation is proved between the surface area and PHRR because they showed the same trend. Foams having the same density but different surface area and chemical composition show great variation of PHRR (up to 50%) both at 75 and 50kW/m2. 

We have created pilot installation for CVD production of multiwall carbon nanotubes from ethylene in industrial scale. MWNT produced by this process have average diameter 12-20 nm, surface area near 200-500 m2/g, mass content of minerals 6-20% for non-purified NT and <1% for purified NT. Electron images of MWNT samples with different surface area (200, 390, and 500 m2/g) are shown in Figs. 1-3. 

Cb) Transformation curves of anatase samples of two different surface areas at 700°C Si, 55 m2/g... 

Surface area is one of the most important characteristics in particle technology. Particles with a different surface area will express different physical properties that will subsequently affect many applications and ultimately final dosage form properties. 

The influence of the first factor was diminished by working at a low temperature in an atmosphere of hydrogen. The employment of two copper coulometers in series fitted with cathodes of different surface-areas, and the determination of the slight difference between the weights of the copper deposits, made it possible to extrapolate for the increase in weight of a cathode of zero-area, and thus to eliminate the solution-error. 

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