Catalytic reactions enhancement factor

Figure 4.23. Comparison of predicted and measured enhancement factor A values for some of the early studies of catalytic reactions found to exhibit the NEMCA effect.1,19 Reprinted with permission from Elsevier Science.1...

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

It is rare that a catalyst can be chosen for a reaction such that it is entirely specific or unique in its behaviour. More often than not products additional to the main desired product are generated concomitantly. The ratio of the specific chemical rate constant of a desired reaction to that for an undesired reaction is termed the kinetic selectivity factor (which we shall designate by 5) and is of central importance in catalysis. Its magnitude is determined by the relative rates at which adsorption, surface reaction and desorption occur in the overall process and, for consecutive reactions, whether or not the intermediate product forms a localised or mobile adsorbed complex with the surface. In the case of two parallel competing catalytic reactions a second factor, the thermodynamic factor, is also of importance. This latter factor depends exponentially on the difference in free energy changes associated with the adsorption-desorption equilibria of the two competing reactants. The thermodynamic factor also influences the course of a consecutive reaction where it is enhanced by the ability of the intermediate product to desorb rapidly and also the reluctance of the catalyst to re-adsorb the intermediate product after it has vacated the surface. 

This chapter is intended to focus on catalysis in both thermal and photoinduced electron transfer reactions between electron donors and acceptors by investigating the effects of an appropriate substance that can reduce the activation barrier of electron transfer reactions. It is commonly believed that a catalyst affects the rate of reaction but not the point of equilibrium of the reaction. Thus, a substance is said to act as a catalyst in a reaction when it appears in the rate equation but not in the stoichiometric equation. However, autocatalysis involves a product acting as a catalyst. In this chapter, a catalyst is simply defined as a substance which affects the rate of reaction. This is an unambiguous classification, albeit not universally accepted, including a variety of terms such as catalyzed, sensitized, promoted, accelerated, enhanced, stimulated, induced, and assisted. Both thermal and photochemical redox reactions which would otherwise be unlikely to occur are made possible to proceed efficiently by the catalysis in the electron transfer steps. First, factors that accelerate rates of electron transfer are summarized and then each mechanistic viability is described by showing a number of examples of both thermal and photochemical reactions that involve catalyzed electron transfer processes as the rate-determining steps. Catalytic reactions which involve uncatalyzed electron transfer steps are described in other chapters in this section [66-68]. 

Of all the functions of proteins, catalysis is probably the most important. In the absence of catalysis, most reactions in biological systems would take place far too slowly to provide products at an adequate pace for a metabolizing organism. The catalysts that serve this function in organisms are called enzymes. With the exception of some RNAs (ribozymes) that have catalytic activity (described in Sections 11.7 and 12.4), aU other enzymes are globular proteins (section 4.3). Enzymes are the most efficient catalysts known they can increase the rate of a reaction by a factor of up to 10 ° over uncatalyzed reactions. Non-enzymatic catalysts, in contrast, typically enhance the rate of reaction by factors of 102 to 104... 

For enhancement factors higher than nnity the reaction is called non-Faradaic. The rate enhancement factor, p, is defined as the ratio of the promoted catalytic rate, r, to the initial open-circnit reaction rate, r, and it is a measnre of the level of promotion ... 

The open-circuit enhancement factor, y, indicates the reversibility of the electrochemical promotion, and is defined as the ratio of two steady-state open-circuit catalytic reaction rates, one after polarization, r , and the other before polarization, i-o -... 

Note that, unlike in the definition of effectiveness factor for catalytic reactions (Chapter 7) where the normalizing rate was the rate of reaction, here the normalizing rate is the rate of mass transfer. Thus the reaction is considered the intruder (albeit benevolent or enhancing), whereas for catalytic reactions, diffusion was the intruder (often, but not always, retarding). For a pseudo-mth-order reaction, one can write... 

Catalytic reactions constitute a special type of reaction mechanism in which a chemical amplification takes place. With very low catalyst concentrations a significant amount of reaction product can be measured, enhancing the analytical sensitivity of the measurements. In general, these reactions are not amplification reactions. However, in some enzymatic reactions, a series of enzymes and other co-factors act on each other in a sequential fashion, providing a cascade reaction or, in a cyclic process, providing a rapid, amplified response of the small initial signal, and these processes can be considered examples of amplification reactions. 

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