Electrochemical oxidation supporting electrolytes

Anodic Oxidation. The abiUty of tantalum to support a stable, insulating anodic oxide film accounts for the majority of tantalum powder usage (see Thin films). The film is produced or formed by making the metal, usually as a sintered porous pellet, the anode in an electrochemical cell. The electrolyte is most often a dilute aqueous solution of phosphoric acid, although high voltage appHcations often require substitution of some of the water with more aprotic solvents like ethylene glycol or Carbowax (49). The electrolyte temperature is between 60 and 90°C. 

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

FIG. 2 Cyclic voltammogram of the ferricenium transfer across the water-DCE interface at lOmVs. The electrochemical cell featured a similar arrangement to Fig. 1(b), but the organic phase contained 2mM of ferrocene. Heterogeneous oxidation of Fc occurred in the presence of 0.2mM CUSO4 in the aqueous phase. Supporting electrolytes were lOmM 02804 and lOmM BTPPATPBCl. The transfer of the standard tetramethylammonium (TMA+) under the same condition is also superimposed. 

The electrochemical reduction reactions of the central metallotetraphenylporphyrin moieties are, fortunately, much more straightforwardly analyzed (1,2). With few exceptions, when transferred to a fresh supporting electrolyte solution, films formed from ECP reactions like Fig. 2A exhibit electrochemical reduction waves at or very near the potentials observed for reductions of the corresponding monomers dissolved in solutions. For example, a film formed oxidatively as in Fig. 2A gives in fresh electrolyte the reductive gyclic voltammogram of Fig. 2B. 

The products of electrochemical oxidation of conjugated dienes are considerably affected by the reaction conditions such as the material of the electrode, the supporting electrolyte and the solvent. The oxidation of butadiene with a graphite or carbon-cloth anode in 0.5 M methanolic solution of NaClCU mainly yields dimerized products along with small amounts of monomeric and trimeric compounds (equation 5)1. The use of platinum or glassy carbon mainly gives monomeric products. Other dienes such as isoprene, 1,3-cyclohexadiene, 2,4-hexadiene, 1,3-pentadiene and 2,3-dimethyl-l,3-butadiene yield complex mixtures of isomers of monomeric, dimeric and trimeric compounds, in which the dimeric products are the main products. 

Ouweltjes JP, van Berkel FPF, Nammensma P, and Christie GM. Development of 2nd generation, supported electrolyte, flat plate SOFC components at ECN. In Singhal SC, Dokiya M, editors. Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells (SOFC-VI). Pennington, NJ The Electrochemical Society, 1999 99(19) 803-811. 

Chemical reactivity of unfunctionalized organosilicon compounds, the tetraalkylsilanes, are generally very low. There has been virtually no method for the selective transformation of unfunctionalized tetraalkylsilanes into other compounds under mild conditions. The electrochemical reactivity of tetraalkylsilanes is also very low. Kochi et al. have reported the oxidation potentials of tetraalkyl group-14-metal compounds determined by cyclic voltammetry [2]. The oxidation potential (Ep) increases in the order of Pb < Sn < Ge < Si as shown in Table 1. The order of the oxidation potential is the same as that of the ionization potentials and the steric effect of the alkyl group is very small. Therefore, the electron transfer is suggested as proceeding by an outer-sphere process. However, it seems to be difficult to oxidize tetraalkylsilanes electro-chemically in a practical sense because the oxidation potentials are outside the electrochemical windows of the usual supporting electrolyte/solvent systems (>2.5 V). 

Electrochemical studies of the behaviour of [100] (5 X 10 4 mol dm-3 in dichloromethane solution containing 0.1 mol dm-3 [NBu"]BF4 as supporting electrolyte) have been carried out using cyclic and square-wave voltammetric techniques. The receptor itself undergoes two quasi-reversible oxidations at Epi = +350 mV and Ep2 = +450 mV referenced to Ag/Ag+. Rotating disk... 

In the usual experiment for the electrochemical oxidation of methanol, this reactant is added to a supporting electrolyte. When an electrochemical measurement is executed under this condition, oxidation of both bulk methanol and adsorbed species are mixed up and analyses are difSciiit... 

The direct electrochemical oxidation of aliphatic alcohols (1) to carbonyl compounds (2) (Eq. 1) is not a convenient way for synthesis because of the high oxidation potentials of alcohols. The oxidation always competes with the oxidation of a solvent and supporting electrolyte, leading to low current efhdencies and side products. 

It has been found that the electrochemically generated NO radical addes to the substituted olefins 81, and the radical species 81a formed is further oxidized to the cationic intermediate 81b which reacts with acetonitrile and yields 82 (Scheme 41). The anodic oxidation was carried out in a mixed solvent CH3CN-Et20 with NaNOa as a supporting electrolyte. The oxazoline derivatives 82 were isolated in 69-77% yield [103],... 

Electrochemically, the BF4 anion was found to be stable against oxidation on a glassy carbon (GC) surface up to 3.6 V vs a standard calomel electrode (SCE), which translates into - 5.0 V vs lithium. When a distinction is made, this stability limit is somehow lower than those of AsFe and PFe anions however, caution must be exercised here, as these data were measured on GC with quaternary ammonium as supporting electrolyte, instead of on a surface of cathode materials. This could result in substantial difference. ... 

The simplest design of electrochemical cell has two electrodes dipping into the solution containing the substrate and the supporting electrolyte. A cell of this type is suitable for the Kolbe oxidation of carboxylate ions (see p. 316) where the anode reaction is given by Equation 1.1 and the cathode reaction is the evolution of hydrogen (Equation 1.2). Both the substrate and the hydrocarbon product are inert... 

Electrochemical reactions require a solvent and electrolyte system giving as small a resistance as possible between the anode and cathode. Erotic solvents used include alcohol-water and dioxan-water mixtures and the electrolyte may be any soluble salt, an acid or a base. Duiing reaction, protons are consumed at the cathode and generated at die anode so that a buffer will be required to maintain a constant pH. Aprotic solvents are employed for many reactions [18], the most commonly used being acetonitrile for oxidations and dimethylforraamide or acetonitrile for reductions. In aprotic solvents, the supporting electrolyte is generally a tetra-alkylammonium fluoroborate or perchlorate [19], Tlie use of perchlorate salts is discouraged because of the possibility that traces of perchlorate in the final product may cause an explosion. 

Butadienes give a complex mixture of methoxylated products by electrochemical oxidation in methanol with sodium perchlorate as supporting electrolyte [44]. Dimethoxybutenes are formed together with dimers from reaction of medioxybu-tenyl radicals. A platinum anode gives the highest yields of monomeric products while graphite anodes yield only dimeric products. This is a distinction from the... 

Electrochemical oxidation of aldoximes using halide ions as mediators afforded the corresponding nitrile oxides in the anode compartment, which were simultaneously reduced to nitriles by cathodic reduction (equation 15). Sodium chloride affords the best result among the supporting electrolytes (Cl > Br > 1 > C104 > TsO ). Accordingly, the electrochemical reaction of oximes carried out in the presence of dipolephiles yielded isooxazolines (equation 16). 

I. 4-methoxyacetophenone (30 //moles) was added as an internal standard. The reaction was stopped after 2 hours by partitioning the mixture between methylene chloride and saturated sodium bicarbonate solution. The aqueous layer was twice extracted with methylene chloride and the extracts combined. The products were analyzed by GC after acetylation with excess 1 1 acetic anhydride/pyridine for 24 hours at room temperature. The oxidations of anisyl alcohol, in the presence of veratryl alcohol or 1,4-dimethoxybenzene, were performed as indicated in Table III and IV in 6 ml of phosphate buffer (pH 3.0). Other conditions were the same as for the oxidation of veratryl alcohol described above. TDCSPPFeCl remaining after the reaction was estimated from its Soret band absorption before and after the reaction. For the decolorization of Poly B-411 (IV) by TDCSPPFeCl and mCPBA, 25 //moles of mCPBA were added to 25 ml 0.05% Poly B-411 containing 0.01 //moles TDCSPPFeCl, 25 //moles of manganese sulfate and 1.5 mmoles of lactic acid buffered at pH 4.5. The decolorization of Poly B-411 was followed by the decrease in absorption at 596 nm. For the electrochemical decolorization of Poly B-411 in the presence of veratryl alcohol, a two-compartment cell was used. A glassy carbon plate was used as the anode, a platinum plate as the auxiliary electrode, and a silver wire as the reference electrode. The potential was controlled at 0.900 V. Poly B-411 (50 ml, 0.005%) in pH 3 buffer was added to the anode compartment and pH 3 buffer was added to the cathode compartment to the same level. The decolorization of Poly B-411 was followed by the change in absorbance at 596 nm and the simultaneous oxidation of veratryl alcohol was followed at 310 nm. The same electrochemical apparatus was used for the decolorization of Poly B-411 adsorbed onto filter paper. Tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte when methylene chloride was the solvent. 

The possibility of electrochemically producing CgQ anions in a defined oxidation state by applying a proper potential can be used to synthesize fulleride salts by electrocrystallization [39, 75-80]. An obvious requirement for this purpose is the insolubility of the salt in the solvent to be used for the electrocrystallization process. This can be achieved by choosing the proper solvent, the oxidation state of Cjq and the counter cation, which usually comes from the supporting electrolyte. 

The ferrocenyl dendrimers were electrodeposited in their oxidized forms onto the electrode surfaces (platinum, glassy-caibon, and gold) either by controlled potential electrolysis or by repeated cycling between the appropriate anodic and cathodic potential limits therefore the amount of electroactive material electrode-posited can be controlled with the electrolysis time or the number of scans. The electrochemical behavior of films of the polyfeirocenyl dendrimers was studied by cyclic voltammetry in fresh CH2CI2 and CHjCN solutions containing only supporting electrolyte. 

In this solvent, using CV and Osteryoung square-wave voltammetry (OSWV) under high vacuum conditions at room temperature, Cgo displays a one-electron, chemically reversible oxidation wave at +1.26 V vs. Fc/Fc+. TBAPFe was used as the supporting electrolyte. Under the same conditions, the first one-electron oxidation of C70 occurs at +1.20 V, 60 mV more negative (easier to oxidize) than that of Cgo- Both oxidations are electrochemically quasireversible with A pp = 80 mV. In addition, a second oxidation wave is observed for C70 close to the limit of the solvent potential window at+1.75 V. However, this wave appears to be chemically irreversible (see Fig. 3) [36]. 

The kinetics of the electrochemical oxidation of ammonia on platinum to dinitrogen in basic electrolytes has been extensively studied. In the widely supported mechanism originally suggested by Gerischer and Mauerer[ll], the active intermediate in the selective oxidation to N2 is a partly dehydrogenated ammonia adsorbate, NH2 ads or NHaatomic nitrogen adsorbate N ag, which is apparently formed at more positive potentials, is inactive toward N2 production at room temperature. Generally, only platinum and iridium electrodes exhibit steady-state N2 production at potentials at which no sur-... 

---