Electrochemical-chemical mechanism

This mixed chemical/electrochemical mechanism has been critized by Randin [41], who points out that the main characteristics of the process are accommodated by a number of mechanisms, including the one described. 

The electrochemical oxidation of model compounds of NADH in acetonitrile gives two main oxidation peaks in the absence of a base, clearly demonstrating a stepwise oxidation of NADH (41). An ECE (electrochemical-chemical-electrochemical) mechanism for the electrochemical oxidation of NADH has been proposed in several studies (41-47) most often depicted ... 

Based on the analysis of the obtained data the ECE (electrochemical-chemical-electrochemical) mechanism of C02 electroreduction can be suggested ... 

The reactions of polyaromatic hydrocarbons (PAHs) in DDAB bicontinuous microemulsions followed the ECE (electrochemical-chemical-electrochemical) mechanism at low scan rates [81,82]. The three-step ECE process consists of electron transfer to the PAH to form an anion radical, protonation of the anion radical to yield a neutral radical,... 

Electrochemical-Chemical-Electrochemical mechanism. Heterogeneous electron transfer Electron transfer Ethyl... 

The electrochemical oxidation of NADH can be described as a two single electron and a one proton transfer (electrochemical-chemical-electrochemical mechanism - ECE) [5]. The mechanism is shown in Fig. 3. 

The mechanism for electrochemical oxidation of NADH has been proposed in a number of studies as an electrochemical-chemical-electrochemical mechanism characterized by the following reaction scheme (Equation 4.1) [32-34] ... 

Modified surface A surface that has properties different from the bulk, and in which the bulk material is detectable in the modified surface. Surface modification can be done chemically, electrochemically, mechanically, etc. Examples Anodized aluminum shot-peened... 

Magnesium is employed ki a wide variety of appHcations, based on its chemical, electrochemical, physical, and mechanical properties. The International Magnesium Association (IMA) divides the markets for magnesium kito 10 categories and tracks the volume of primary magnesium shipments to each market area on an annual basis. 

We conclude that corrosion is a chemical reaction (equation 10.1) occurring by an electrochemical mechanism (equations 10.2) and (10.3), i.e. by a process involving electrical and chemical species. Figure 10.1 is a schematic representation of aqueous corrrosion occurring at a metal surface. 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

TABLE 2-1 Electrochemical Mechanisms Involving Coupled Chemical Reactions... 

The overall two-electron mechanism has been generally reported to obey an ECE (Electrochemical-Chemical-Electrochemical) process where the chemical reaction is the generally fast chemical scission of the C—S bond of the sulphone anion radical. 

Using impedance data of TBN+ adsorption and back-integration,259,588 a more reliable value of <7 0 was found for a pc-Cu electrode574,576 (Table 11). Therefore, differences between the various EffM) values are caused by the different chemical states and surface structures of pc-Cu electrodes prepared by different methods (electrochemical or chemical polishing, mechanical cutting). Naumov etal,585 have observed these differences in the pzc of electroplated Cu films prepared in different ways. 

The purpose of this paper Is 1) to describe the electrochemistry of ferrl-/ferro-cyanlde and the oxidation of ascorbic at an activated glassy carbon electrode which Is prepared by polishing the surface with alumina and followed only by thorough sonlcatlon 2) to describe experimental criteria used to bench-mark the presence of an activated electrode surface and 3) to present a preliminary description of the mechanism of the activation. The latter results from a synergistic Interpretation of the chemical, electrochemical and surface spectroscopic probes of the activated surface. Although the porous layer may be Important, Its role will be considered elsewhere. 

Corrosion (from Latin corrodere, gnaw to pieces ) of metals is the spontaneous chemical (oxidative) destruction of metals under the elfect of their environment. Most often it follows an electrochemical mechanism, where anodic dissolution (oxidation) of the metal and cathodic reduction of an oxidizing agent occur as coupled reactions. Sometimes a chemical mechanism is observed. 

The individual steps of the multistep chemical reduction of COj with the aid of NADPHj require an energy supply. This supply is secured by participation of ATP molecules in these steps. The chloroplasts of plants contain few mitochondria. Hence, the ATP molecules are formed in plants not by oxidative phosphorylation of ADP but by a phosphorylation reaction coupled with the individual steps of the photosynthesis reaction, particularly with the steps in the transition from PSII to PSI. The mechanism of ATP synthesis evidently is similar to the electrochemical mechanism involved in their formation by oxidative phosphorylation owing to concentration gradients of the hydrogen ions between the two sides of internal chloroplast membranes, a certain membrane potential develops on account of which the ATP can be synthesized from ADP. Three molecules of ATP are involved in the reaction per molecule of COj. 

The electrochemical mechanism was rejected by Salvago and Cavallotti [26] on the basis that it does not explain several features of electroless deposition of ferrous metals it does not account for the isotopic composition of the H2 gas evolved it does not explain the effect of the various solution components on reaction rate and it does not account for the homogeneous decomposition of very active solutions or the fact that they can give deposition on insulating surfaces. These authors put forward a chemical mechanism, involving various hydrolyzed nickel species, which they claim explains the observed behavior of the system ... 

This mechanism is based on the known importance of hydroxides in other deposition reactions, such as the anomalous codeposition of ferrous metal alloys [38-39], Salvago and Cavallotti claim an analogy with the mechanism of Ni2 + reduction from colloids in support of their proposed mechanism. There is no direct evidence for the hydrolyzed species, however. Furthermore, the mechanism does not explain two experimentally observed facts Ni deposition will proceed if the Ni2 + and the reducing agent are in separate compartments of a cell [36, 37] and P is not deposited in the absence of Ni2 +. The chemical mechanism does not take adequate account of the role of the surface state in catalysis of the reaction. It has no doubt been the extreme oversimplification, by some, of the electrochemical mechanism that has led other investigators to reject it. 

Mital et al. [40] studied the electroless deposition of Ni from DMAB and hypophosphite electrolytes, employing a variety of electrochemical techniques. They concluded that an electrochemical mechanism predominated in the case of the DMAB reductant, whereas reduction by hypophosphite was chemically controlled. The conclusion was based on mixed-potential theory the electrochemical oxidation rate of hypophosphite was found, in the absence of Ni2 + ions, to be significantly less than its oxidation rate at an equivalent potential during the electroless process. These authors do not take into account the possible implication of Ni2+ (or Co2+) ions to the mechanism of electrochemical reactions of hypophosphite. 

In order to understand the methodology in some detail, we first consider homogeneous processes, where the electrochemical techniques used are well-established. Such processes are not central to this book, which is primarily concerned with electrode processes, but they do serve to illustrate the manner in which mechanisms can be explored. As indicated above, any step in the electrochemical mechanism must be either chemical (denoted by C) or electro-chemical (denoted by E) in nature. It is not normally the case that more than one electron is transferred simultaneously, so possible sequences may be written down straightforwardly. 

As mentioned previously, a large number of redox reactions involving macrocyclic ligand complexes have resulted in discrete changes in the unsaturation pattern of a variety of macrocyclic systems. Chemical, electrochemical, and catalytic reactions have been widely used to change the level of unsaturation in such systems. Although the mechanisms of the majority of such transformations are not well understood, it is clear that the reactions tend to proceed via prior change in the oxidation state of the central metal ion. 

In the third region of coverage, most of the atomic scale roughness has been proposed to be irreversibly destroyed as the Pb layer rearranges to assume the final hexagonal close packed configuration of the monolayer.( ) This loss of atomic scale roughness results in the irreversible decrease in i/(0H) intensity to essentially unmeasurable levels. This observation further emphasizes the importance of the chemical enhancement mechanism contribution to SERS in electrochemical systems. 

This requirement can be fulfilled by reduction of the electron-acceptor unit(s) or by oxidation of the electron-donor unit(s) by chemical, electrochemical, or photochemical redox processes. In most cases, the CT interaction can be restored by an opposite redox process, which thus promotes a reverse mechanical movement leading to the original structure. 

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