Oxidation-reduction reactions practical applications

Oxidoreductases comprise a large class of enzymes that catalyze biological oxidation/reduction reactions. Because so many chemical transformation processes involve oxidation/reduction processes, the idea of developing practical applications of oxidoreductase enzymes has been a very attractive, but quite elusive, goal for many years [83], Applications have been sought for the production of pharmaceuticals, synthesis and modification of polymers, and the development of biosensors for a variety of clinical and analytical applications [83], In recent years, the use of oxido-reductive enzymes to catalyze the removal of aromatic compounds from... 

Batteries are a practical application of the galvanic cell in that an oxidation-reduction reaction generates an electric current. A battery that has an enormous impact on our lives is the automobile battery, shown in Figure 10.3. 

Gray, Harry B., John D. Simon, and William C. Trogler. Braving the Elements. Sausalito, Calif. University Science Books, 1995. This book is an introduction to the basic principles of chemistry, with elementary explanations of radioactive decay, chemical bonding, oxidation-reduction reactions, and acid-base chemistry. Practical applications of specific chemical compounds and classes of compounds are presented. 

All refined metals have a tendency to revert to a thermodynamically more stable form such as those in which they occur naturally on earth. Thus one of the corrosion products of iron is iron oxide (Fea03(s)) which is one form of iron ore. Almost all types of corrosion can be explained in terms of electrochemistry (oxidation-reduction reactions) for this reason we will consider corrosion as an example of the application of redox chemistry or electrochemistry to a practical situation. We will not present a detailed quantitative analysis of corrosion and the design of corrosion-control systems. Other texts should be consulted for this type of information. 

Based on our earlier discussions, we know that Eh can be measured as cell potential in reference to SHE. The question is, can we measure e activity in a similar manner as the measurement of H+ activity. In the absence of temperature, pe is a simple hypothetical calculation and represents transformation of Eh. However, pe values have much narrower range than Eh values, and can be easily combined with pH values (see discussion in the latter part of this chapter). Although there are advantages in using the pe concept in handling oxidation-reduction reactions, electrons do not occur as an independent species in the solution, and it is questionable to think of their activity. From a practical standpoint, a potential in volts is probably simpler and has a wider applicability. 

Oxidation-reduction reactions involve energy changes. Because these reactions involve electron transfer, the net release or net absorption of energy can occur in the form of electrical energy rather than as heat. This property allows for a great many practical applications of redox reactions. The branch of chemistry that deals with electricity-related applications of oxidation-reduction reactions is called electrochemistry. 

For alkaline electrolytes, the oxidizer reduction reaction (ORR) kinetics are more efficient than acid-based electrolytes (e.g., PEFC, PAFC). Many space appUcations utiUze pure oxygen and hydrogen for chemical propulsion, so the AFC was well suited as an APU. However, the alkaline electrolyte suffers an intolerance to even small fractions of carbon dioxide (CO2) found in air which react to form potassium carbonate (K2CO3) in the electrolyte, gravely reducing performance over time. For terrestrial applications, CO2 poisoning has limited lifetime of AFC systems to well below that required for commercial application, and filtration of CO2 has proven too expensive for practical use. Due to this limitation, relatively little commercial development of the AFC beyond space applications has been realized. Some recent development of alkaline-based solid polymer electrolytes is underway, however. The AFC is discussed in greater detail in Chapter 7. 

Practical applications of oxidation-reduction reactions can be traced back thousands of years to the period in human culture when metal tools were first made. The metal needed to make tools was obtained by heating copper or iron ores, such as cuprite (CU2O) or hematite (Fe203), in the presence of carbon. Since that time, iron has become the most widely used of all metals and it is produced in essentially the same way by heating Fe203 in the presence of carbon in a blast furnace. A simplified chemical equation for the reaction is given below. 

In this chapter, we will see how chemical reactions can be used to produce electricity and how electricity can be used to cause chemical reactions. The practical applications of electrochemistry are countless, ranging from batteries, fuel cells, and biological processes to the manufacture of key chemicals, the refining of metals, and methods for controlling corrosion. Before we can understand such applications, we must first discuss how to carry out an oxidation-reduction reaction in an electrochemical cell and explore how the energy obtained from, or supplied to, an electrochemical cell is related to the conditions under which the cell operates. 

This simple concept has already found some practical applications The idea to use supported alkali-promoted noble metal catalysts for NO reduction,3,4 even under mildly oxidizing conditions,5 came as a direct consequence of electrochemical promotion studies utilizing both YSZ (Chapter 8) and p"-Al203 (Chapter 9), which showed clearly the electrophi-licity of the NO reduction reaction even in presence of coadsorbed O. This dictated the use of a judiciously chosen alkali promoter coverage to enhance both the rate and selectivity under realistic operating conditions on conventional supported catalysts. 

Many investigators use pulse techniques in which a catalyst reacts with hydrocarbons, oxygen etc. separately in time. This can provide an insight into the nature and significance of the individual reaction and sorption steps, but it should be emphasized that selectivities and other data may be unrepresentative for conditions in a flow reactor. In particular, selectivities may be considerably lower under steady state conditions. If the selectivity differences between pulse and flow experiments are very large, a cyclic mode of operation may be attractive for the practical application of the catalyst concerned. Oxidation and reduction are then separated. 

The advantages of PTC reactions are moderate reaction conditions, practically no formation of by-products, a simple work-up procedure (the organic product is exclusively found in the organic phase), and the use of inexpensive solvents without a need for anhydrous reaction conditions. PTC reactions have been widely adopted, including in industrial processes, for substitution, displacement, condensation, oxidation and reduction, as well as polymerization reactions. The application of chiral ammonium salts such as A-(9-anthracenylmethyl)cinchonium and -cinchonidinium salts as PT catalysts even allows enantioselective alkylation reactions with ee values up to 80-90% see reference [883] for a review. Crown ethers, cryptands, and polyethylene glycol (PEG) dialkyl ethers have also been used as PT catalysts, particularly for solid-liquid PTC reactions cf. Eqs. (5-127) to (5-130) in Section 5.5.4. 

Electrometric Titration Precipitation Reactions.—One of the most important practical applications of electrode potentials is to the determination of the end-points of various typos of titration the subject will be treated here from the standpoint of precipitation reactions, while neutralization and oxidation-reduction processes are described more conveniently in later chapters. 

Calcinm glectrowinning in piutniiium production. A potential application of inorganic membranes in radioactive waste treatment is in the industrially practiced direct oxide reduction process. In this process plutonium oxide is calciothermit y reduced to plutonium in the presence of calcium chloride according to the following reaction ... 

The constant potential amperometric detector determines the current generated by the oxidation or reduction of electoactive species at a constant potential in an electrochemical cell. Reactions occur at an electrode surface and proceed by electron transfer to or from the electrode surface. The majority of electroactive compounds exhibit some degree of aromaticity or conjugation with most practical applications involving oxidation reactions. Electronic resonance in aromatic compounds functions to stabilize free radical intermediate products of anodic oxidations, and as a consequence, the activation barrier for electrochemical reaction is lowered significantly. Typical applications are the detection of phenols (e.g. antioxidants, opiates, catechols, estrogens, quinones) aromatic amines (e.g. aminophenols, neuroactive alkaloids [quinine, cocaine, morphine], neurotransmitters [epinephrine, acetylcoline]), thiols and disulfides, amino acids and peptides, nitroaromatics and pharmaceutical compounds [170,171]. Detection limits are usually in the nanomolar to micromolar range or 0.25 to 25 ng / ml. 

A homogeneous precatalyst may be active because it forms a heterogeneous catalyst under the reaction conditions. This is particularly likely with metal ions such as Pd(II) that are readily reduced to the metal itself. In such a case, a colloidal heterogeneous catalyst may be the true active species. A number of tests have been proposed to test for this situation (16). Nanoparticles resulting from such reduction procedures have been carefully characterized (17). In oxidation processes, metal oxides may form and be the true active species. For practical applications, this may be of little consequence but it may lead to completely erroneous mechanistic interpretations and hinder attempts to improve the catalyst by rational means. 

Thus far in our discussion of electrolysis, we have encountered only electrodes that were inert they did not undergo reaction but merely served as tlie surface where oxidation and reduction occurred. Several practical applications of electrochemistry, however, are based on active electrodes—electrodes that participate in the electrolysis process. Electroplating, for example, uses electrolysis to deposit a ihin layer of one metal on another metal in order to improve beauty or lesistarKe io corrosion (Figure 20.29 T). We can illustrate the principles of electrolysis with active electrodes by describing how to electroplate nickel on a piece of steel. 

Proton exchange membranes (PEMs) are one of the key materials in low-temperature fuel cells proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMECs). Especially, recent trend in the research and development of low-temperamre fuel cells focuses on PEMFCs for transportation (electric vehicle) applications due to the impact on economy and environment. The most important role of PEMs is to transport protons formed as a product of oxidation reaction of fuels at the anode to the cathode, where oxygen reduction reaction takes place to produce water. In addition to this, there are a number of requirements for PEM materials for the practical fuel cell applications, which include... 

---