Catalysis and External Transfer Processes

Physical transport processes can play an especially important role in heterogeneous catalysis. Besides film diffusion on the gas/liquid boundary there can also be diffison of the reactants (products) through a boundary layer to (from) the external surface of the solid material and additionally diffusion of them through the porous interior to from the active catalyst sites. Heat and mass transfer processes influence the observed catalytic rates. For instance, as discussed previously the intrinsic rates of catalytic processes follow the Arrhenius... 

One of the distinctions of electrocatalysts, which differs from that of conventional heterogeneous catalysis, is that the electron transfer processes between the oxidant and the reductant are separated into two half-reactions which are carried out in separate reaction zones. This enables the transfer of electrons through an external electrical circuit which can potentially power a load by doing useful work. An obvious advantage of such electrochemical cells is that it becomes possible to use different types of catalyst materials for the respective half-cell reactions and can be subjected to different types of environments. 

In a typical catalytic reaction, the electron transfer between reactant and catalyst is localized, thus, it cannot introduce the electron from external electrocircuit and cannot also export the electron from reaction system to form current during the reaction process. Moreover, the electron transfer cannot be controlled by external force for catalytic reaction. On the contrary, there is net electron transfer during the electrode reaction. The electrode, a heterogeneous catalyst, is a local site not only for reaction, but also for electron donation-acceptance. This means that electro catalysis possesses the bi-properties of chemical reaction and electron transfer simultaneously. 

Catalysis at interfaces between two immiscible liquid media is a rather wide topic extensively studied in various fields such as organic synthesis, bioenergetics, and environmental chemistry. One of the most common catalytic processes discussed in the literature involves the transfer of a reactant from one phase to another assisted by ionic species referred to as phase-transfer catalyst (PTC). It is generally assumed that the reaction process proceeds via formation of an ion-pair complex between the reactant and the catalyst, allowing the former to transfer to the adjacent phase in order to carry out a reaction homogeneously [179]. However, detailed comparisons between interfacial processes taking place at externally biased and open-circuit junctions have produced new insights into the role of PTC [86,180]. 

The discussed mechanisms represent a form of intramolecular catalysis of the oxidation of the FeII(CN)5 or Run(edta) centers by the Ruii(NH3)5 moiety. The first two moieties react sluggisly and, on the other hand, the electron in RuII(NH3)5 is readily accessible to the external oxidant and is given up. The rapid electronic isomerization processes aid in the consumption of the full oxidation process. This is not truly catalytic because the catalyst is the reactant itself, which, of course, is consumed in the reaction. A better description involves a net oxidation of the FeII(CN)5 or Run(edta) sites through activation by the facile intramolecular electron transfer between the metal centers. The mechanism is described in Fig. 23, bearing some resemblance to the classical chemical mechanism for inner sphere electron... 

In succinyl derivatives, the freely rotating single bond between carbon 2 and 3 allows the terminal carboxyl group to assume many more orientations and as a result this drastically reduces the probability that it will stay in the proper conformation long enough for deacylation to occur. Kirby et al. (46) proposed an intramolecular catalysis of amide bond hydrolysis by a proton transfer from external general acids and also showed that in dilute acid the O-protonated amide is the reactive species that initiates the deacylation process (see Reaction 3). 

An important aspect of the function of photosynthetic complexes is their asymmetric arrangement in respect to the membrane and to the external and internal phases of the cellular compartments. This arrangement allows the catalysis of vectorial electron transfer and the performance of electrical work by promoting charge separation across the membrane dielectric barrier. It allows also in some cases the net translocation of protons across the membrane. These two processes are at the basis of the mechanism of energy conservation in photosynthesis coupled to the formation of ATP, which is added, in oxygenic photosynthesis, to the conservation of redox energy in the form of reduced pyridine nucleotide coenzymes. 

The catalytic behavior of enzymes in immobilized form may dramatically differ from that of soluble homogeneous enzymes. In particular, mass transport effects (the transport of a substrate to the catalyst and diffusion of reaction products away from the catalyst matrix) may result in the reduction of the overall activity. Mass transport effects are usually divided into two categories - external and internal. External effects stem from the fact that substrates must be transported from the bulk solution to the surface of an immobilized enzyme. Internal diffusional limitations occur when a substrate penetrates inside the immobilized enzyme particle, such as porous carriers, polymeric microspheres, membranes, etc. The classical treatment of mass transfer in heterogeneous catalysis has been successfully applied to immobilized enzymes I27l There are several simple experimental criteria or tests that allow one to determine whether a reaction is limited by external diffusion. For example, if a reaction is completely limited by external diffusion, the rate of the process should not depend on pH or enzyme concentration. At the same time the rate of reaction will depend on the stirring in the batch reactor or on the flow rate of a substrate in the column reactor. 

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