Activation surface area Increase

Figure 9.17 Local biofibn potential (black) and redox potential (gray) for (a) the low-conductivity case with interactions allowed and (b) the low-conductivity case with the electrochemically active surface area increased 100 times.

Since catalyst activity is dependent on how much catalytically active surface is available, it is usually desirable to maximi2e both the total surface area of the catalyst and the active fraction of the catalytic material. It is often easier to enlarge the total surface area of the catalyst than to increase the active component s surface area. With proper catalyst design, however, it is possible to obtain a much larger total active surface area for a given amount of metal or other active material in a supported catalyst than can be achieved in the absence of a support. 

Surface Area. This property is of paramount importance to catalyst performance because in general catalyst activity increases as the surface area of the catalyst increases. However because some reaction rates are strongly dependent on the nature of the stmcture of the catalytic surface, a linear correlation of catalyst activity with surface area should not be expected. As the catalyst surface area increases, for many reactions the selectivity of the catalyst is found to decrease. If the support material is completely inert to the reactants and products, this effect may be diminished somewhat. 

Trend changes in catalyst activity, surface area, rare earth, and metals content. Consider adding/increasing metals inhibitor. 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

Adsorption versus Polymerization. It is instructive to examine further the time dependence of the corrosion inhibition. In acid corrosion inhibition tests, steady state is customarily assumed to be reached within 10 to 20 min after initial exposure of the metal specimen. Since the inhibitors function by reducing the available active surface area, we expect an increase in and a corresponding decrease in P. The degree of corrosion protection the inhibitor provides is given by... 

This facilitates the flow of degradable species through these tunnels onto the surface of the TiC>2 where electron could be donated to the holes of the anatase phase and the photocatalytic action in combination with the cavitational effect of the ultrasound can accelerate the fragmentations of pollutants. The details of this mechanism are however discussed at the end of chapter. Ultrasound also breaks TiC>2 particles to still smaller size and increases the active surface area manifold. 

This phenomenon, however, is not difficult to understand in view of the mechanism of dissolution under such conditions. Since the number of active sites increases linearly with current density and these sites are characterized by a film structure (or thickness or both) different from that at the OCP, one could expect corresponding increases in the corrosion rate. However, as was mentioned earlier, the active surface area in the pits increases with time, and hence one should expect the corrosion rate to increase correspondingly. Therefore, since the effect is not time dependent, one... 

Varying KOH ratio in the mixture is a very effective way of controlling porosity development in resultant activated carbons. The trend in the pore volume and BET surface area increase seems to be similar for various precursors (Fig. la). It is interesting to note, however, a sharp widening of pores, resulting in clearly mesoporous texture, when a large excess of KOH is used in reaction with coal semi-coke (Fig. lb). Increase in the reaction temperature within 600-900°C results in a strong development... 

It has been suggested [21,22] that the presence of Cu and K increases the rates and extent of Fe304 carburization during reaction and the FTS rates, by providing multiple nucleation sites that lead to the ultimate formation of smaller carbide crystallites with higher active surface area. In the present investigation, Cu- and K-promoted iron catalysts performed better than the unpromoted catalysts in terms of (1) a lower CH4 selectivity, (2) higher C5+ and alkene product selectivi-ties, and (3) an enhanced isomerization rate of 1-alkene. 

The meniscus is quite small if the funnel is kept still, and partitioning is slow. Conversely, shaking the funnel generates a large number of small globules of solvent, which gready increases the active surface area of the meniscus. Therefore, we shake the funnel to increase the rate of partitioning. 

If we accept a date of around AD 1000 for the commencement of the distillation of zinc on a large scale, then, following the work of Craddock (1978), all earlier brasses should contain less than 28% Zn, as this is the approximate upper limit for the calamine process at around 1000 °C. Above this temperature, the process is more efficient, but it is said that the brass produced melts and the active surface area for the process is thus reduced. By granulating the copper and therefore increasing the surface area, the maximum can be pushed to around 33% Zn, but it is unlikely that this was done in Europe until the 18th Century (see Section 6.4). This model is supported by the analytical data Craddock s work on Roman brass indeed shows an upper limit of about 28% zinc. 

Thus, the surface provides the chances for reactants to combine and form activated complex and accelerate the rate of reaction. An increase in the surface area (increasing peaks, cracks, corners) of the catalyst increases the rate of reaction. 

In the case of solid interfaces which are in the form of coarse powders, cavitation collapse can produce enough energy to cause fragmentation and activation through surface area increase. For very fine powders the partides are accelerated to high velocity by cavitational collapse and may collide to cause surface abrasion (Fig. 3.5). For some metal powders these collisions generate sufficient heat to cause particle fusion. 

Active po vder can been produced from molten zinc, less than 100 pm in size, at a rate of 1 kg/h under sonication [74], Dispersed palladium, platinum and rhodium prepared by sonochemical reduction of aqueous salts also exhibit a higher reactivity, assigned to a surface area increase [75],... 

An analogous case would be when the solid is crushed and the surface area increases per unit gram (Figure 1.5). For example, finely divided talcum powder has a surface area of 10 m2/g. Active charcoal exhibits surface areas corresponding to over 1000 m2/g. This is obviously an appreciable quantity. Qualitatively, one must notice that work has to be put into the system when one increases the surface area (both for liquids or solids or any other interface). 

The anode layer of polymer electrolyte membrane fuel cells typically includes a catalyst and a binder, often a dispersion of poly(tetraflu-oroethylene) or other hydrophobic polymers, and may also include a filler, e.g., acetylene black carbon. Anode layers may also contain a mixture of a catalyst, ionomer and binder. The presence of a ionomer in the catalyst layer effectively increases the electrochemically active surface area of the catalyst, which requires a ionically conductive pathway to the cathode catalyst to generate electric current (16). 

Concluding this section, when very fine particles were dispersed in the reactant solution, i.e., the number of particles and the surface area increased, the reactivity of the sonophotocatalytic reaction decreased and the product ratio became lower. In general, for photocatalytic reactions, the finer the photocatalyst, the better for the reaction. However, for sonophotocatalytic reactions it was found that the finer the particles such as Ti02-B in the reactant solution, the worse the product ratio. Since it is impractical to obtain and use a photocatalyst of very large particle size to increase the activity limitlessly, a suitable particle size must be selected to obtain high performance in the sonophotocatalytic reaction. 

Enhancement of the activity of Ag has been observed upon continuous potential cycling [265], as well as with more complex potential sequences including potential holding at some cathodic values [266] and pulsating overpotential [267]. The enhancement cannot be explained only in terms of surface area increase, so that the creation of especially active Ag sites has been postulated [266]. 

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