Silver-catalyzed electrodes

Nickel and Liu [79] have studied the silver-catalyzed oxidation of A, A -dimethyl-/7-phenylenediamine by Co(NH3)5Cl3 by means of a silver colloid and a rotating silver electrode. They conclude that the reaction can be quantitatively explained by means of the theory of mixture potential and mixture current of Wagner and Traud... 

Copper(II) and cerium(IV) have been studied as oxidants in acetonitrile. The copper(II)-copper(I) couple has an estimated electrode potential of 0.68 V relative to the silver reference electrode. It has been studied as an oxidant for substances such as iodide, hydroquinone, thiourea, potassium ethyl xanthate, diphenylbenzidine, and ferrocene. Cerium(IV) reactions are catalyzed by acetate ion. Copper(I) is a suitable reductant for chromium(VI), vanadium(V), cerium(IV), and manganese(VII) in the presence of iron(III). For details on many studies of redox reactions in nonaqueous solvents, the reader is referred to the summary by Kratochvil. ... 

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

Conditions existing at the triple interface, Ag/AgX/Solution, will influence the rate at which the catalyzed reaction occurs. These conditions will determine the activity of the silver ions, the concentration of adsorbed developer, and the state of the catalyst. The halide ion will be an important factor in determining the activity of the silver ions, and Sheppard has suggested that the hydration and diffusion away of the halide ion is the dominant factor in determining the specific rate of reaction at the interface (Sheppard, 15). The known order of reactivity of the halides, AgCl > AgBr > Agl, follows as a natural consequence from this point of view, whereas it would not be predicted on the basis of the electrode mechanisms. 

The modified electrode, in turn, enhances Mb to catalyze the reduction of H2O2. Addition of H202 increases in the cathodic peak intensity for the modified electrode. This electrochemical response is specific for the Mb silver NP modified electrode because no signals are observed for either the bare electrode or an electrode with only a silver NP film. A Michaelis-Menten analysis demonstrates that the modified electrode shows higher affinity for H202 than other gold-cytochrome c and gold-horseradish peroxidase modified electrodes. The apparent enhanced catalytic activity... 

Enzymes are often employed in the chemical layer to impart the selectivity needed. We saw an example of this in Chapter 13 when discussing potentiometric enzyme electrodes. An example of an amperometric enzyme electrode is the glucose electrode, illustrated in Figure 15.4. The enzyme glucose oxidase is immobilized in a gel (e.g., acrylamide) and coated on the surface of a platinum wire cathode. The gel also contains a chloride salt and makes contact with silver-silver chloride ring to complete the electrochemical cell. Glucose oxidase enzyme catalyzes the aerobic oxidation of glucose as follows ... 

Impressed current cathodic protection requires (i) DC power supply (rectifier) (ii) an inert anode such as catalyzed titanium anode mesh (iii) wiring conduit (iv) an embedded silver/silver chloride reference electrode. A schematic diagram of an impressed current cathodic system is shown in Figure 5.26. By an impressed current, the potential of the steel is adjusted to values greater than -850 mV, thus making the steel bar cathodic and prevent the corrosion (25). 

The auxiliary electrodes intended to facilitate the reduction of oxygen comprise of, for example, carhon—teflon porous mass catalyzed with silver [6,7] or phthalocyanine [8]. Ruetschi and Ockermann have established that both hydrogen oxidation and oxygen reduction can proceed on the same auxiliary electrode [9]. In our institute, we developed auxiliary catalytic electrodes with tungsten carbide (WC) or a mixture of WC and active carbon as catalyst [10—12], Figure 14.4 presents the volt-ampere curves of the hydrogen and oxygen reactions on partially immersed auxiliary electrodes with different catalysts WC alone, or WC plus active carbon (WC-I-C), or platinum (Pt) [10-12]. 

Direct, nonmediated electrochemical reduction of NADIP)" " at modified electrode surfaces has been used to produce the en2ymatically active NAD(P)H and even to couple the NAD(P)H regeneration process with some biocatalytic reactions [228]. The modifier molecules used for these purposes are not redox active and they do not mediate the electron-transfer process between an electrode and NAD(P)+ however, they can effectively decrease the required overpotential and prevent formation of the nonenzymatically active dimer product [228]. For example, the efficiency of the direct electrochemical regeneration of NADH from NAD" " was enhanced by the use of a cholesterol-modified gold amalgam electrode that hinders the dimerization of the NAD-radicals on its modified-surface [228]. This direct electrochemical NAD+ reduction process was used favorably to drive an enzymatic reduction of pyruvate to D-lactate in the presence of lactate dehydrogenase. The turnover number for NAD" " was estimated as 1400 s k Other modifiers that enhance formation of the enzymatically active NAD(P)H include L-histidine [229] and benzimidazole [230], immobilized as monolayers on silver electrodes. CycKc voltammetric experiments demonstrated that these modified electrodes can catalyze the reduction of NAD+ to enzymatically active NADH at particularly low overpotentials. 

The Siemens cell was similar except that the air electrode was fabricated with two layers a hydrophilic layer of porous nickel on the electrolyte side for oxygen evolution and a hydrophobic layer [carbon black bonded with Teflon (PTFE) and catalyzed with silver] on the air side for oxygen reduction. The dual porosity helped to shield the silver catalyst from oxidation. As many as 200 cycles were achieved. ... 

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