Electrochemical reaction classification

Table 4.1 lists all published electrochemical promotion studies of 58 catalytic reactions on the basis of the type of electrolyte used. Each of these reactions is discussed in Chapters 8 to 10 which follow the same reaction classification scheme. 

C.G. Vayenas, S. Brosda, and C. Pliangos, Rules and Mathematical Modeling of Electrochemical and Chemical Promotion 1. Reaction Classification and Promotional Rules,/. Catal., in press (2001). 

Crystalline solids are built up of regular arrangements of atoms in three dimensions these arrangements can be represented by a repeat unit or motif called a unit cell. A unit cell is defined as the smallest repeating unit that shows the fuU symmetry of the crystal structure. A perfect crystal may be defined as one in which all the atoms are at rest on their correct lattice positions in the crystal structure. Such a perfect crystal can be obtained, hypothetically, only at absolute zero. At all real temperatures, crystalline solids generally depart from perfect order and contain several types of defects, which are responsible for many important solid-state phenomena, such as diffusion, electrical conduction, electrochemical reactions, and so on. Various schemes have been proposed for the classification of defects. Here the size and shape of the defect are used as a basis for classification. 

Nonaqueous Solvents. Many organic compounds are not soluble in water, and the investigator who desires to study their electrochemistry must resort to organic solvents. The solvents most often used are the so-called dipolar aptotic solvents that belong to Class 5a in the classification scheme of Table 7.5. These are solvents with moderately large dielectric constants and low proton availability. This aptotic character tends to simplify the electrochemical reactions often the primary product is a stable radical cation or anion that is produced by removal or addition of an electron. 

Electrode processes are conveniently classified according to the nature of the final product1 and its formal mode of formation, since then the interplay between nucleophile(s) or electrophile(s), substrate, and loss or addition of electron(s) is best expressed. It is upon our ingenuity to choose the correct combination of electrolyte components that the practical success of an electrochemical reaction rests, and therefore the rather formalized classification system to be outlined and exemplified below is the logical point of departure into the maze of mechanistic intricacies of electrode processes. 

Classification of electrochemical reactions j = A exp(-AGac/kT) Vrm- coupling constant with the electrode... 

Figure 2. Classification of electrochemical reactions according to the strength of the interaction with the electrode.

Note that this classification is done for discharges where the current is carried by electrons. For electrochemical discharges this is the case when the active electrode is a cathode the electrons travel through the gas film from the electrode to the electrolyte, where they will somehow undergo electrochemical reactions with the ions of the electrolyte. It is harder to imagine that this would also be the case if the active electrode is an anode. Indeed, this would raise serious questions about how electrons would be emitted from the electrolyte and travel to the anode. As discussed in Section 2.5, the situation is very different for this case. For the time being, we restrict the discussion to the situation where the active electrode is a cathode. 

A common classification of inhibitors is based on their effects on the electrochemical reactions involved in the corrosion process. In the framework of mixed potential theory (see Chapter 1.3, this volume), these effects are most conveniently visualized by -log j I diagrams, such as shown in Fig. 1. For a freely corroding metal, the corrosion potential Eq and the corrosion... 

Fuel cells are electrochemical devices that convert chemical energy contained in fuel directly into electrical energy through electrochemical reactions. Fuel cells consist of an anode, where the fuel is oxidized, a cathode where the oxidant is reduced and an electrolyte which separates anode from cathode and conducts ions. The general classification of fuel cells is usually based on the type of electrolyte used, and their operation conditions are typically related to the characteristics of the electrolyte. More detailed discussion of fuel cells as stand-alone power sources can be found in the next chapter of this book. 

The variety of nature, composition and structure of the compounds available as active materials causes a diversity of lithiation/de-lithiation electrochemical reactions. A classification can be established on the basis of the nature of the redox reaction that occurs in the active material to accommodate the lithium. This is what is done below. 

Classification by the type of attack At room temperature most corrosion reactions are triggered by electrochemical reactions ( electrolytic corrosion ), whereas at higher temperatures metal/gas type reactions will prevail. In addition to chemical/electrochemical attack, friction or a mechanical load will cause specific corrosion reactions. 

Electrochemical Series A classification of the elements according to the values of the standard potentials of specified electrochemical reactions. 

Two different technological approaches are reflected by this classification. The main problem of low-temperature cells consists in finding efficient electrocatalysts so that the rates of the electrochemical reactions are still satisfactory at low polarization. Since the reaction rates increase with temperature, the role of the electrocatalyst is not so critical in the high-temperature cells. Other problems like corrosion and conductivity of the electrolyte become pertinent for the construction of a practical unit. 

Table 4.2. Classification of electrochemical promotion studies on the basis of catalytic reaction.

Table 4.3. Classification of Electrochemical Promotion studies on the basis of catalytic reaction, showing the observed kinetic order with respect to the electron donor (D) and electron acceptor (A) reactant and the corresponding global rvs

Figure 2.1 Classification of electrochemical electron transfer reaction on metal electrodes. (See color insert.)...

All of these reactions types can be effected at the cathode or the anode. However, since organic electrode processes often occur via a blend of radical and ionic mechanisms, it should be stressed that the simplicity of the classification scheme above does not imply that the electrochemical oxidation or reduction of a given substrate results in only one type of reaction on the contrary, many processes give products emanating from two or more of the reaction types above, and it may often involve a lot of experimental work to find optimum conditions for a (desired) reaction to occur, if it is at all possible. 

A second mode of classification of electrode reactions is based entirely on the electrode mechanism. Here it is necessary to know the number of chemical and electrochemical steps involved and the order between the different steps. By denoting an electrochemical step by an if and a chemical one by a C and postulating that every electrochemical step involves the transfer of one electron, it is immediately evident that, e.g., an ECEC process consists of ... 

Classification by End Use Chemical reactors are typically used for the synthesis of chemical intermediates for a variety of specialty (e.g., agricultural, pharmaceutical) or commodity (e.g., raw materials for polymers) applications. Polymerization reactors convert raw materials to polymers having a specific molecular weight and functionality. The difference between polymerization and chemical reactors is artificially based on the size of the molecule produced. Bioreactors utilize (often genetically manipulated) organisms to catalyze biotransformations either aerobically (in the presence of air) or anaerobically (without air present). Electrochemical reactors use electricity to drive desired reactions. Examples include synthesis of Na metal from NaCl and Al from bauxite ore. A variety of reactor types are employed for specialty materials synthesis applications (e.g., electronic, defense, and other). 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

Because of the difficulties discussed above, our classification of the reactions of HNCC is based on the products obtained from those reactions. They are discussed in six separate sections A. Oxidation Reactions, B. Protonation and Deprotonation Reactions, C. Reduction Reactions, D. Electron-Transfer Reactions and Electrochemical Studies, E. Reactions with Soft Nucleophiles, and F. Oxidative Addition of the Small Molecules H2,12, and HX. 

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