Electrodes as Catalysts

COUPLING OF ELECTRODE ELECTRON TRANSFERS WITH CHEMICAL REACTIONS 

More complicated mechanisms of the same category are encountered in SrnI reactions (Section 2.5.6) where the electrocatalytic reaction, which corresponds to a zero-electron stoichiometry, is opposed to two-electron consuming side reactions (termination step in the chain process). 

Numerical Computations. Simulations. Diagnostic Criteria. Working Curves 

There are three levels of increasing difficulty in computing the mathematical expressions defining, in dimensionless terms, the current responses in cyclic voltammetry or with any other analytical techniques. The simplest case is that of an analytical expression. This is found, for example, for a Nernstian 

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Wagner, 7 Wolkenstein, 279 Work function and absolute potential, 353 and electrochemical promotion, 138 and cell potential, 138, 218 Helmholtz equation, 24 of metals, 139 measurement of, 138 spatial variations, 222 variation with coverage, 24 Working electrode as catalyst, 9 overpotential of, 123... 

Metalloporphyrins consist of porphyrin ring structures complexed to a central atom. Among them, hemin structures with central iron atoms at different oxidation states and chlorophyll pigments containing magnesium are most abundant The interest in their spectroelectrochemical studies is multiple. Thus, their adsorption and electrochemical behaviour at the electrode surface can be used not only to model their functions in a biological matrix but also to improve the practical application of porphyrin coated electrodes as catalysts or sensitizers in photoelectrochemical cells... 

A very few organic chemicals have been produced commercially by electrochemical means. In recent years many more have been synthesized in laboratories, in large part because of improved control of potential plus better insight into electrodes as catalysts. Controlled electrochemical steps in commercial organic synthesis are now clearly in prospect. 

Fuel cells essentially reverse the electrolytic process. Two separated platinum electrodes immersed in an electrolyte generate a voltage when hydrogen is passed over one and oxygen over the other (forming H30+ and OH-, respectively). Ruthenium complexes are used as catalysts for the electrolytic breakdown of water using solar energy (section 1.8.1). 

An obvious extension of the bipolar design idea presented in the previous section is the induction of NEMCA using multi-stripe or multi-dot Pt catalysts placed between two terminal Au electrodes, as shown in Figs. 12.8 and 12.9. Both designs have been successfully tested as shown in these figures.10 Larger terminal voltages are applied here between the two Au electrodes, so that the potential difference in each individual cell formed between the Pt stripes or dots is of the order of IV.10... 

Two types of continuous flow solid oxide cell reactors are typically used in electrochemical promotion experiments. The single chamber reactor depicted in Fig. B.l is made of a quartz tube closed at one end. The open end of the tube is mounted on a stainless steel cap, which has provisions for the introduction of reactants and removal of products as well as for the insertion of a thermocouple and connecting wires to the electrodes of the cell. A solid electrolyte disk, with three porous electrodes deposited on it, is appropriately clamped inside the reactor. Au wires are normally used to connect the catalyst-working electrode as well as the two Au auxiliary electrodes with the external circuit. These wires are mechanically pressed onto the corresponding electrodes, using an appropriate ceramic holder. A thermocouple, inserted in a closed-end quartz tube is used to measure the temperature of the solid electrolyte pellet. 

Structure The polymers are produced as powders or as films on the electrodes. Most conductive polymers have a fibrous structure, each fiber consisting of hundreds of strands of polymer molecules. Techniques exist to control fiber preparation so as to obtain nanofibers expected to be particularly useful as catalyst substrates and in electronic applications (MacDiannid, 2000). 

In conclusion, therefore, at least for metal catalysts it will not be justihed to identify crystallographic defects emerging at the electrode surface with the active sites responsible for the catalytic activity of the electrode as a whole. 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

Conventional colloid chemistry and elaitrochemistry have always been clo ly related with each other, the keywords electrophoresis, double layer theory, and specific adsorption describing typical asp ts of this relationship. In more ro nt times, new aspects have arisen which again bring colloid chemistry into contact with modem developments in electrcolloidal particles as catalysts for electron transfer reactions and as photocatalysts. In fact, the similarity between the reactions that occur on colloidal particles and on compact electrodes has often been emphasized by calling the small particles microelectrodes . 

Kinetic results such as those presented in the previous sections, which could be further extended by varying the reaction parameters (reactant concentration, electrode potential, catalyst loading, electrolyte flow rate, and reaction temperature), can serve as basis... 

Figure 15.3 Simulated effectiveness factor for porous carbon electrode as a function of the exchange current density jo and DCo for Ip] = 0.4 V for a 10wt% Pt/C catalyst layer with 7= 10, A = 140m g p = 2gcm, Nafion volume fraction 0.6, thickness p,m, and ionic conductivity 0.05 Scm See the text for details. (Reproduced from Gloaguen et al. [1994], with kind permission from Springer Science and Business Media.)...

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

It is found that some types of active carbons possess enough catalytic activity to be used as catalysts in air electrodes operating at low c.d. 

In Figure 4 we have presented the experimental Tafel plots of air electrodes with catalysts from pure active carbon and from active carbon promoted with different amounts of silver. The obtained curves are straight lines with identical slopes. It must be underlined that the investigated electrodes possess identical gas layers and catalytic layers, which differ in the type of catalyst used only. Therefore, the differences in the observed Tafel plots can be attributed to differences in the activity of the catalysts used. The current density a at potential zero (versus Hg/HgO), obtained from the Tafel plots of the air electrodes is accepted as a measure of the activity of the air gas-diffusion electrodes the higher value of a corresponds to higher activity of the air electrode. 

Figure 6 presents the polarization curves of one and the same electrode with active carbon as catalyst when operating with air and with pure oxygen. 

Active carbon promoted with small amount of silver is used as catalyst in the air electrodes of these cells. In Figure 15 we presented the discharge curve of the zinc-air cell ZV3000 at constant current 1 A. 

This reaction is reversible on a platinized Pt electrode and radical formation is the key to reversibility. Nonelectrodic homolysis does not take place unless platinum black is present as catalyst. The evidence is based on considerable exchange between isotopic 210Pb-labeled PhgPb2 and Ph3PbN03 which only occurs in the presence of platinum black. A similar case has been shown for PhgSn25(). 

In the simplest catalytic reaction scheme (Figure 2.16) a fast and reversible couple, P/Q serves as catalyst (mediator) for the reduction (taken as an example, transposition to oxidation being straightforward) of the substrate A. Instead of taking place at the electrode surface, electron transfer to A occurs via... 

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