Reaction rate increase, surface phases

The functions of porous electrodes in fuel cells are 1) to provide a surface site where gas/liquid ionization or de-ionization reactions can take place, 2) to conduct ions away from or into the three-phase interface once they are formed (so an electrode must be made of materials that have good electrical conductance), and 3) to provide a physical barrier that separates the bulk gas phase and the electrolyte. A corollary of Item 1 is that, in order to increase the rates of reactions, the electrode material should be catalytic as well as conductive, porous rather than solid. The catalytic function of electrodes is more important in lower temperature fuel cells and less so in high-temperature fuel cells because ionization reaction rates increase with temperature. It is also a corollary that the porous electrodes must be permeable to both electrolyte and gases, but not such that the media can be easily "flooded" by the electrolyte or "dried" by the gases in a one-sided manner (see latter part of next section). 

The change from inhibition to acceleration of the rate of electrode reaction occurs exactly at the mixed solvent composition at which the process of reactant resolvation begins in the bulk of the solution. It appears that even partial resolvation of vanadium(III) ions (Avv(in)>0) initiates the increase of the rate of reaction in the surface phase, when the concentration of the organic solvent is considerably higher there than in the bulk. Such behavior is observed in mixtures of water with solvents of Lewis basicity lower (AN) and also higher (HMPA) than water. 

Figure 5.2. Nucleation and growth in a solid state reaction. The measure (area) of the interface between the reactant and the product phase increases at first when the interface grows and the reaction rate increases. Near the end of the reaction when the interface diminishes the reaction rate decreases. The reaction occurs at the surface and is proportional to its measure.

Sodium cyanide does not dissolve m butyl bromide The two reactants contact each other only at the surface of the solid sodium cyanide and the rate of reaction under these con ditions IS too slow to be of synthetic value Dissolving the sodium cyanide m water is of little help because butyl bromide is not soluble m water and reaction can occur only at the interface between the two phases Adding a small amount of benzyltrimethyl ammonium chlonde however causes pentanemtnle to form rapidly even at room temper ature The quaternary ammonium salt is acting as a catalyst it increases the reaction rate How7... 

For many laboratoiy studies, a suitable reactor is a cell with independent agitation of each phase and an undisturbed interface of known area, like the item shown in Fig. 23-29d, Whether a rate process is controlled by a mass-transfer rate or a chemical reaction rate sometimes can be identified by simple parameters. When agitation is sufficient to produce a homogeneous dispersion and the rate varies with further increases of agitation, mass-transfer rates are likely to be significant. The effect of change in temperature is a major criterion-, a rise of 10°C (18°F) normally raises the rate of a chemical reaction by a factor of 2 to 3, but the mass-transfer rate by much less. There may be instances, however, where the combined effect on chemical equilibrium, diffusivity, viscosity, and surface tension also may give a comparable enhancement. 

Increase in interfacial area. The total surface area for diffusion is increased because the bubble diameter is smaller than for the free-bubbling case at the same gas flow rate hence there is a resultant increase in the overall absorption rate. The overall absorption rate will also increase when the diffusion is accompanied by simultaneous chemical reaction in the liquid phase, but the increase in surface area only has an appreciable effect when the chemical reaction rate is high the absorption rate for this case is then controlled by physical diffusion rather than by the chemical reaction rate (G6). 

In heterogeneous liquid/liquid reactions, cavitational collapse at or near the interface will cause disruption and mixing, resulting in the formation of very fine emulsions. When very fine emulsions are formed, the surface area available for the reaction between the two phases is significantly increased, thus increasing the rates of reaction. The emulsions formed using cavitation, are usually smaller in size and more stable, than those obtained using conventional techniques and often require little or no surfactant to maintain the stability [8]. This is very beneficial particularly in the case of phase-transfer catalyzed reactions or biphasic systems. 

In gas phase reactions an increase in the surface to volume ratio of the reaction vessel may reduce the reaction rate, while addition of inert gases may increase the reaction rate. (See Section 4.2.5.)... 

Enhanced chemical reactivity of solid surfaces are associated with these processes. The cavitational erosion generates unpassivated, highly reactive surfaces it causes short-lived high temperatures and pressures at the surface it produces surface defects and deformations it forms fines and increases the surface area of friable solid supports and it ejects material in unknown form into solution. Finally, the local turbulent flow associated with acoustic streaming improves mass transport between the liquid phase and the surface, thus increasing observed reaction rates. In general, all of these effects are likely to be occurring simultaneously. 

---