Mass transport catalytic cycle

The second mechanism often invoked to explain the increase in n y of simple Fe porphyrins at potentials more reducing than that of the Fe couple (under anaerobic conditions) is based on the fact that at such potentials the fraction of the catalyst in the 5 -coordinate ferrous state is maximal because (i) the equilibrium (18.9) is shifted completely to the ferrous form and (ii) the concentration of O2 in the catalytic film is low owing to mass transport limitations. The higher the concentration of the 5-coor-dinate ferrous porphyrin in the catalytic film, the greater the probability that any released H2O2 will re-enter the catalytic cycle by coordinating to a molecule of ferrous porphyrin and decay according to (18.13b) instead of (18.17). 

When interfacial electron exchange rate(s) are sufficiently high and the response is free from mass transport hmitations, the catalytic current will be determined by the inherent activity of the enzyme. Variation of current (activity) with potential can be explained by an extension of the Michaelis-Menten description of enzyme kinetics that relates activity to oxidation state through incorporation of the Nemst equation." " The resulting expressions describe the catalytic cycle, and include rates of intramolecular electron exchange, chemical events, substrate binding and product release, together with the reduction potentials of centres in the enzyme, and the influence of... 

If it is intended to study the reaction kinetics, that is, the dependence rate upon variables such as reaction pressures or temperature, it is important to be sure that the surface reaction is the slowest of all those forming the catalytic cycle, and that therefore mass transport to and from the surface are not rate limiting. Symptoms of mass transport limitation include the following. 

In the previous chapters we predominantly considered catalysis as a molecular event, in which substrate molecules are activated by the catalyst. In this chapter and the next we will emphasize catalytic features of dimensions in space much larger than that of single catalytic centers and times much longer than those associated with the individual molecular catalytic cycles. Often mass and heat transport cause reaction cycles, which occur at different sites, to interact. Under particular conditions this gives rise to cooperative phenomena with oscillatory kinetics and temporal spatial organization. As such, interesting surface patterns such as spirals or pulsars may form. Such complex cooperative phenomena are known in physics as appearances of excitable systems. Their characteristic features are easily influenced by small variations in external conditions. Hence these systems have also features that are called adaptive. 

The intrinsic catalytic cycle contains only the chemical steps 3-4-5 of the 7-step sequence listed in the so-called continuous reaction model. It is necessary to make the assumption of zero gradients with respect to heat and mass transport both outside and within the catalyst particle. Therefore, experimental conditions in the laboratory have to be adjusted to ensure that (i) external transport processes (steps 1 and 7 of the sequence) are very rapid compared to chemical steps and (ii) internal transport processes (steps 2 and 6 of the sequence) are negligible, that is, particle sizes are small enough to ignore pore structure. In Figure 2.3, the reactant concentration profile labeled as IV represents the case for intrinsic kinetics. 

The GOx and mediator, M, may be anchored to the surface or free in solution. Since this is a catalytic cycle, significant enhancement factors are observed in the current measurement at the electrode which, combined with the advantageous properties of nanoelectrodes and their arrays (high mass transport, fast electron transfer, low capacitance) leads to a highly sensitive detection technique. 

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