Kinetics anodic oxidation

In conclusion, one can say that most anodic oxide films are of a duplex, or even triplex, character, with only the inner portion being composed of a pure anhydrous oxide. In the duplex films, the outer layer contains anions and often a degree of hydration. There could exist a third thin oxide layer at the surface, again with somewhat different properties, which may have a role in the kinetics of oxide growth. 

Figure 31. Correlation of the kinetics of oxide growth with kinetics of sulfate incorporation into the oxide during galvanostatic (a) and potentio-static (b) anodization of A1 in Ff2S04 solutions.166...

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

Anodic oxide films formed under different kinetic conditions vary in structure, composition and property (e.g., etch rate) and they change with time during the anodization. 

The second current maximum J3 corresponds to an oxide thickness at which tunneling of charge carriers becomes negligible, as shown in Fig. 4.7. At the bias corresponding to J3 the formation of anodic oxides in electrolyte-free HF shows a change of growth kinetics, as shown in Fig. 5.2. 

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

CI2 evolution reaction, 38 56 electrochemical desorption, 38 53-54 electrode kinetics, 38 55-56 factors that determine, 38 55 ketone reduction, 38 56-57 Langmuir adsorption isotherm, 38 52 recombination desorption, 38 53 surface reaction-order factor, 38 52 Temkin and Frumkin isotherm, 38 53 real-area factor, 38 57-58 regular heterogeneous catalysis, 38 10-16 anodic oxidation of ammonia, 38 13 binding energy quantification, 38 15-16 Haber-Bosch atrunonia synthesis, 38 12-13... 

The combination of anodic oxidation of benzene using the Ag(I)/Ag(II) mediator with cathodic oxidation of benzene using the Cu(I)/Cu(II) mediator in a single electrolytic cell produces p-benzoquinone selectively in both the anodic and the cathodic chambers [242]. Silver-mediator promoted electrooxidation of hydrocarbon has been attempted [243]. The kinetics of indirect oxidation of catechol and L-dopa with IrCl6 has been studied in polymer-coated glassy carbon [244]. 

Combined with methanol crossover, slow anode kinetics lead to a power density of a DMFC that is three to four times lower than that of a hydrogen fuel cell. Much work has been focused on the anodic oxidation of methanol. The mechanism of the... 

The thermodynamic stabilities of phenonium ions have been determined based on bromide-transfer equilibria in the gas phase and, depending on the substituents, the bridged species (1) has been proposed as an intermediate or transition state on the potential-energy surface for the 1,2-aryl rearrangement of triarylvinyl cations (see Scheme 1). Phenonium ion (3) has been presented as an intermediate to account for the fact that lactonization of methyl 4-aryl-5-tosyloxy hexanoate (2) produces y-lactone (4) selectively under thermodynamic conditions, but affords 5-lactone (5) preferentially under kinetic conditions. It has been shown that anodic oxidation of frany-stilbene in alcohols in the presence of KF or BU4NBF4 is accompanied by its electro-oxidative rearrangement into diphenylacetaldehyde acetals. The mechanism outlined in Scheme 2 has been proposed" for the transformation. 

Wendt, H. and Plzak, V. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 2. Electrocatalysis and electrocatalysts for cathodic evolution and anodic oxidation of hydrogen, in Electrochemical Hydrogen Technologies (ed. H. Wendt), Elsevier, Amsterdam, Chapter 1. 2. 

Hwang BJ, Hwang JR (1993) Kinetic model of anodic oxidation of titanium in sulphuric acid. J Appl Electrochem 23 1056-1062... 

The anodic oxidation of 10-phenylphenothiazine in CH3CN-Et4NC104 solution of pyridine gives rise to the formation of JV-[3-(10-phenylphenothia-zinyl)]pyridinium perchlorate, the parent compound, and pyridinium perchlorate.274 On the basis of kinetic determinations a half-regeneration mechanism was proposed. 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

The results of this kinetic analysis have been included in Table I. It can be seen that, if both the anodic decomposition of the semiconductor and the anodic oxidation of the competing reactant would occur by irreversible hole-capture steps ((L)(H)(I) or (M)(H)(1)), as was hitherto generally accepted, the stabilization should be independent of light intensity, in contradiction with the results described above. The mechanism in which the reducing agent reacts by donating an electron to a localized surface hole ((L)(X)) leads to an expression in which s is a function of the variable (y/j) only. The three other mechanisms considered lead to the relationship of the type (18), in which s is a function of (y2/j). 

Diamond surfaces after anodic oxidation treatment involve oxygen-containing surface functional groups. The electron-transfer kinetics for ions and polar molecules are expected to be quite different. Fe(CN)l /4 was highly sensitive to the surface termination of diamond. For an anionic reactant, there was an inhibition of the electron transfer for the oxygen-terminated diamond electrodes compared with the hydrogen-terminated diamond electrodes, and there was also an acceleration of the electron transfer for oxygen-terminated diamond for some cationic reactants such as Ru(NH3) +/3+ and Fe2+/3+. These results can be explained by electrostatic effects, which interact between the ionic... 

The physico-chemical approach to organic electrode processes has been summarized in a textbook by Conway 19- and further elaborated in reviews on the anodic oxidation of organic compounds 20- and on electrode kinetic aspects of the Kolbe reaction 21 The series of monographs edited by Delahay and Tobias 22-> and Bard 23- also contain many articles of great interest in this connection. 

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