The Need for High Surface Area

Methods of producing such electrodes have been developed over the years. The best substrate is based on carbon, which can produce electrodes having a specific area as high as 2.5 x 10 cm g ). Such materials could be mixed with Pt or Pt-Rh alloys, for example, to prepare similarly high surface area catalysts. 

There have been attempts to determine the catalytic activity of single nanoparticles, as a function of size. This is a delicate matter, since the area of such particles is of the order of 10 cm, and any error in the measurement (or better, the estimation) of the surface area could lead to a major error, when extrapolated to a macroscopic-sized electrode. The same applies to any background or stray current, bearing in mind that a background current of 1 x 10 A could lead to an error of 0.1 A cm or more. 

In addition, the volume-to-surface area ratio in such measurements is extremely high, so that keeping the electrode clean enough, in order to allow a meaningful determination of its inherent catalytic activity, could be an insurmountable challenge, as discussed in Section 7.1.2). 

This leaves the question of the dependence of catalytic activity on the size of the nanopartide open to debate, at least from the point of view of the theory of electrocatalysis. On the other hand, there is no doubt that employing nanoparticles is a valid and highly effective method for producing high-surface area electrodes, thereby increasing the catalytic activity. Whether this should be attributed to an increase in the intrinsic catalytic activity associated with the small size, or just to the increase in electroactive surface area of the electrode may be of secondary importance, from the practical point of view, for example in the design of better anodes in fuel cells. 

---

Here k and k are complex rate constants containing the rate constants associated with all three steps in the isomerisation mechanism above. The important point, though, is that Sr, the total number of active sites at the surface is contained in the rate equation, hence the need for high surface area to maximise Sr- In some cases (especially in selective oxidations) it is necessary to limit Sr and surface area to avoid further reaction/ decomposition of a desired intermediate product. 

Coatings (e.g. paints) applied to metal surfaces can be extremely effective in containing the corrosion of the substrate in many environments. This is particularly true for steel in natural environments. However, no freshly applied coating is entirely free from defects and so there will always be small areas which are exposed directly to the corrosive environment. It is possible to reduce, but not eliminate, these defects by paying attention to workmanship. In practice, it becomes increasingly expensive to achieve fewer and fewer defects because of the need for high grade inspection, and the detection and repair of individual defects. 

As mentioned above, impaired fluid absorption in kidney proximal tubule in AQPl deficiency indicates the need for high cell membrane water permeability for rapid, near-isosmolar fluid transport. The involvement of AQPs in fluid secretion by glands (salivary, submucosal, sweat, lacrimal), and by the choroid plexus and the ciliary body has been investigated using appropriate knockout mouse models. The general conclusion is that AQPs facilitate active fluid (secretion and absorption) when sufficiently rapid, in which case AQP deletion is associated with reduced volume and increased ion/solute content of secreted fluid. AQPs appear not to be needed when fluid secretion rate (per unit epithelial surface area) is low, as AQP-independent water permeability is high enough to support slow fluid secretion (or absorption). 

Fig. 17.17 Conceptual illustration of gas-turbine engine with the combustor sections based on flow through catalyst monoliths. Because of the need for high catalyst surface area, the combustor sections are much larger than those in an ordinary gas turbine based on...

The analysis of the effects of transport on catalysis has focused on a comparison of the availability of reacting species by diffusion to the rate of reaction on the catalytic sites. High-surface-area catalysts are usually porous. Comparison of transport to reaction rates has usually been based on Knudsen diffusion (by constricted collision with the pore walls) as the dominant mode of transport. DeBoer has noted that for small pores surface diffusion may dominate transport (192). Thiele modulus calculations may therefore not be valid if they are applied to systems where surface diffusion can be significant. This may mean that the direct participation of spillover species in catalysis becomes more important if the catalysts are more microporous. Generalized interpretations of catalyst effectiveness may need to be modified for systems where one of the reactants can spill over and diffuse across the catalyst surface. 

The requirement for catalytic surface area may determine in what form the catalyst should be incorporated in the membrane reactor. If the underlying reaction calls for a very high catalytic surface area, the catalyst may need to be packed as pellets and contained inside the membrane tubes or channels rather than impregnated on the membrane surface or inside the membrane pores due to the limited available area or volume in the lauer case. 

Membranes with a high porosity, since the conductive heat transfer coefficient of the gas entrapped within the membrane pores is an order of magnitude smaller than that of the membrane matrix. This possibility is parallel to the need for high DCMD permeability as the available surface area of evaporation is enhanced. 

The development of high surface area electrochemical reactor systems has been one of the most active research and development areas In electrochemical engineering. The demand for systems with high space-time yields has been driven by the-need for economic metal... 

From the great number of oxidoreductases used to modify enzymatic BFC electrodes only a minority is capable of DET, which reduces the number of fuels and oxidants (Table 1). The substrate specificity of enzymes redners half-cell separation by e.g., membranes unnecessary. DET between enzyme and electrode also stops the need for soluble redox mediators to shuttle electrons between enzyme and electrode. This results in the possibility to design membraneless, non-compartmentalized enzymatic BFCs with a simple architecture. However, so far achieved DET currents are lower than MET currents, because usually only enzyme monolayers can be contacted. Strategies to improve the current density aim at the use of high surface area electrode materials like CNTs, AuNPs etc. or the layer-by-layer approach... 

As vessels increase in size the surface area volume ratio decreases thereby increasing the need for high heat transfer rates with the possibility of enhanced encrustation on cooling surfaces or at evaporation zones as a consequence. 

---