Halides, anodic oxidation

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

The importance of the first factor, the concentration, is clear because the anodic photocurrent due to oxidation of halide ions should be proportional to the product of the concentration of positive holes at the electrode surface and that of halide ions in solution. When ip becomes large, the supply of halide ions to the electrode surface by diffusion becomes unable to follow ip, resulting in a decrease of (X ), which depends on the concentration of X". 

Another model for giving an explanation of the pH dependence of the reactivity of halide ions may be that surface cations serve as effective sites for adsorption of reaction intermediates which are produced in the course of the anodic oxidation of halide ions. Usually, the anodic oxidation of halide ions is believed to... 

The potential window can be limited by the decomposition potential of a solute, not just a solvent. In particular, reactions of anodic oxidation of halides (Cl-, Br, and I-) on diamond are highly irreversible and have much higher overvoltage (for Cl, by 1 V) than on platinum or graphite electrodes [97, 123, 124], In all probability this is due to poor adsorption of intermediates, that is, Cl, Br, and I atoms, on the diamond electrode surface. We recall that the outer-sphere reactions discussed in Section 6.1 generally do not involve adsorption of intermediates and thus are not... 

The cationic species formed by the anodic oxidation of halide anions add to alkenes in the presence of suitable nucleopMles (equation 69). ... 

Haiogenation of aromatic nuclei may also be achieved by halogen or positively charged species of halogen formed in solution by anodic oxidation of halide anions (equation 70). ... 

A phenomenon similar to the underpotential deposition of metals is also observed in the study of the anodic oxidation of halides. In Fig. 81 we show the dependence of current on potential during an anodic potential sweep obtained on a platinum electrode in a nonaauenns mpdinm A current peak corresponding to the formation of atomic bromine on the surface is observed, about 0.4 V before the potential for formation of molecular bromine is reached. The two reactions considered ... 

The oxidation of halide ions to molecular halogen is relatively easy consequently, the mechanism of anodic halogenation, brought about by oxidation of organic substrates in the presence of halides is not always clear [249-254]. In many cases it may involve halogenation by anodically generated halogen. In other cases, where the substrate is easily oxidized, the halide has the role of a nucleophile and attacks a radical cation. 

The net result is that metal dissolves from the anode and deposits on the cathode. The phenomenon is the basis of electroplating (e.g., chromium plating of steel), electrowinning, and electrorehning. Also, it is the analytical basis of an electrodeposition method known as electrogravimetry. This involves the separation and weighing of selected components of a sample. Most metal elements can be determined in this manner, usually deposited as the M° species, although some metal elements can be deposited as oxides. The halides can be determined by deposition as the silver halide. Metals commonly determined include Ag, Bi, Cd, Co, Cu, In, Ni, Sb, Sn, and Zn. 

The most common procedure involves the reduction of solution phase metal ions - hydrated or complexed -to elemental metal on the cathode. In the case of anodic deposition of metal as an oxide or halide, the stoichiometry must include any water present. 

In lithium-ion batteries substances should be used as cathode material which can intercalate and discharge lithium ions at a highly positive potential - compared to the intercalation into the carbon anode - and with only low kinetic hindrance, i.e. at low over-voltage or nearly reversible. The first requirement is fulfilled especially by transition metal oxides and halides and also, to a lesser extent, by sulfides. The second requirement of low kinetic hindrance for insertion and release of lithium ions is meant as a requirement of high mobility of lithium ions and electrons within the cathodic lattice and of unhindered mass transfer across phase boundaries as far as phase transitions happen in the host lattice during in- and excorporation of lithium. As the transition metal halides are poorer electronic conductors than oxides, only the latter are used in practice. 

The synthetic potential of electrochemical oxidation protocols for the oxidative phenolic coupling has been discussed earlier for the preparation of colchicine (see Tobinaga s route, Eq. 12.48-1, Scheme 12.48). Vlahov was able to successfully employ an electrochemical protocol as key step in the preparation of galantamine [193], Bromide 314 was utilized in the anodic oxidation. The halide served to block... 

It must be noted that impurities in the ionic liquids can have a profound impact on the potential limits and the corresponding electrochemical window. During the synthesis of many of the non-haloaluminate ionic liquids, residual halide and water may remain in the final product [13]. Halide ions (Cl , Br , I ) are more easily oxidized than the fluorine-containing anions used in most non-haloaluminate ionic liquids. Consequently, the observed anodic potential limit can be appreciably reduced if significant concentrations of halide ions are present. Contamination of an ionic liquid with significant amounts of water can affect both the anodic and the cathodic potential limits, as water can be both reduced and oxidized in the potential limits of many ionic liquids. Recent work by Schroder et al. demonstrated considerable reduction in both the anodic and cathodic limits of several ionic liquids upon the addition of 3 % water (by weight) [14]. For example, the electrochemical window of dry [BMIM][BF4] was found to be 4.10 V, while that for the ionic liquid with 3 % water by weight was reduced to 1.95 V. In addition to its electrochemistry, water can react with the ionic liquid components (especially anions) to produce products... 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

Attempts to prepare In(II) halide complexes by this method are unsuccessful. Anodic oxidation of In metal produces In which disproportionates to the clement and In(II) halide complexes- . 

In this method " - the melt eontains boric oxide and the metal oxide in a suitable electrolyte, usually an alkali or alkaline-earth halide or fluoroborate. The cell is operated at 700-1000 C depending on electrolyte composition. To limit corrosion, the container serving as cathode is made of mild steel or of the metal whose boride is sought. The anode is graphite or Fe. Numerous borides are prepared in this way, e.g., alkaline-earth and rare-earth hexaborides " and transition-metal borides, e.g, TiBj NijB, NiB and TaB... 

The UPD and anodic oxidation of Pb monolayers on tellurium was investigated also in acidic aqueous solutions of Pb(II) cations and various concentrations of halides (iodide, bromide, and chloride) [103]. The Te substrate was a 0.5 xm film electrodeposited in a previous step on polycrystalline Au from an acidic Te02 solution. Particular information on the time-frequency-potential variance of the electrochemical process was obtained by potentiodynamic electrochemical impedance spectroscopy (PDEIS), as it was difficult to apply stationary techniques for accurate characterization, due to a tendency to chemical interaction between the Pb adatoms and the substrate on a time scale of minutes. The impedance... 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

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