Electrocatalysis heterogeneous

However, based strictly on the standard reduction potentials of the two redox couples involved, the equilibrium constant of the reaction as written would be exceedingly small. To further complicate matters, the rates of disappearance of 02 in the presence of Fe(II)TMPyP, as monitored by RRDE techniques, and the rates of disappearance of 02 in the presence of Fe(III)TMPyP, as measured by spectrophotometric methods [52], yielded very similar values. The most likely resolution of this seeming quandary invokes formation of a macrocycle-dioxygen adduct as a short-lived, albeit yet to be detected, intermediate. 

As the electrocatalytic behavior of Fe(III)TMPyP indicates, an increase in the concentration of the mediator leads to a corresponding increase in the net number of electrons transferred to dioxygen. Hence, sizable gains in electrocatalytic efficiency could, in principle, be achieved by immobilizing electrocatalysts on electrode surfaces, as will be discussed in detail in the next section. 

This section describes various strategies for the immobilization of macrocycles on electrode surfaces and their characterization by both electrochemical and in situ spectroscopic techniques in solutions devoid of dioxygen. It also provides theoretical foundations involved in the analysis of the mechanisms of oxygen reduction at such interfaces based on measurements performed under forced convection. Studies involving a number of carefully selected phthalocyanines, and porphyrins, will be presented and discussed, which in our view best illustrate the nuances of the rich behavior this class of adsorbed electrocatalysts can exhibit. These examples serve to 

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G.-Q. Lu, and A. Wieckowski, Heterogeneous Electrocatalysis A Core field of Interfacial Science, Current opinion in Colloid and Interface Science 5, 95 (2000). 

Femandez-Vega A, Feliu JM, Aldaz A, Clavilier J. 1991. Heterogeneous electrocatalysis on well-deflned platinum surfaces modifled by controlled amounts of irreversibly adsorbed adatoms Part IV. Formic acid oxidation on the Pt(lll)-As system. J Electroanal Chem 305 229-240. 

Fig. 34. Schematic presentation of the reaction scheme for heterogeneous electrocatalysis of the second kind catalytic protonation of carbonyl-radical anions by an electrosorbed proton donating surfactant (STR, strychninium).

In order to obtain a reaction rate sufficient for a technical process, the material inside the decomposer should be activated with molybdenum or tungsten carbide instead of simple iron oxide. In this case, the reaction rate using ethanol or propanol to form the corresponding alcoholates is sufficient, too. Higher alcoholates can only be produced in this way with help of a micro-heterogeneous electrocatalysis and/or the help of ultrasonic energy [29-31]. 

Scherson, Palenscar, Tolmachev and Stefan provide a critical review of transition metal macrocycles, in both intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. An introduction is provided to fundamental aspects of electrocatalysis, oxygen reduction, and transition metal macrocydes. Since the theoretical and experimental tools used for investigation of homogeneous and heterogeneous electrocatalysis are considerably different, these topics are given separate discussion. The influence of the electrode surface on adsorbed macrocydes, and their influence on mechanism and rates of 02 reduction is treated in detail. Issues related to pyrolyzed macrocydes are also described. 

The composition of the double layer influences the electron transfer rate (see Sect. 1.3.1.5). Some ions and molecules specifically adsorbed at the eleetrode surface enhance the rate of the electrode process. In such a situation, we talk about heterogeneous electrocatalysis. On the other hand, there are numerous compounds that, after adsorption, decrease the elecdon tfansfer rate and therefore are simply... 

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