Sulfur electrochemical oxidation

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibiHty, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with fuming sulfuric acid (31). A variety of other soluble substituted AJ-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

The main cause of anode wear is electrochemical oxidation or sulfur attack of anodic surfaces. As copper is not sufficiently resistant to this type of attack, thin caps of oxidation and sulfur-resistant material, such as platinum, are bra2ed to the surface, as shown in Eigure 15a. The thick platinum reinforcement at the upstream corner protects against excessive erosion where Hall effect-induced current concentrations occur, and the interelectrode cap protects the upstream edge from anodic corrosion caused by interelectrode current leakage. The tungsten undedayment protects the copper substrate in case the platinum cladding fails. 

Ionic polysulfides dissolve only in media of high polarity hke water, liquid ammonia, alcohols, nitriles, amines, and similar solvents. In all of these solvents 8 can be reduced electrochemically to polysulfide anions. On the other hand, the electrochemical oxidation of polysulfide anions produces elemental sulfur ... 

The various oxidation states of sulfur have been determined by polarography. The electrochemical oxidation of sulfide ions in aqueous solution may lead to the production of elementary sulfur, polysulfides, sulfate, dithionate, and thiosulfate, depending on the experimental conditions. Disulfides, sulfoxides, and sulfones are typical polarographically active organic compounds. It is also found that thiols (mer-captans), thioureas, and thiobarbiturates facilitate oxidation of Hg resulting thus in anodic waves. 

Moscardo-Levelut MN, Plichon V (1984) Sulfur chemistry in equimolar NaOH-HaO melt. I. Electrochemical oxidation of sodium sulfide. J Electrochem Soc 131 1538-1545... 

Fig. 4.13 Electrodeposition of Cd nanoparticles on the graphite surface is followed by electrochemical oxidation and conversion of the oxidized intermediate to CdS or core-shell sulfur-CdS particles. (Reproduced from [125])...

The anodic oxidation of sheet aluminum has been used for a long time to protect aluminum against corrosion by a well-adhering oxide layer. Porous oxide layers are formed if acid electrolytes are used that can redissolve the aluminum oxide (mostly sulfuric or phosphoric acid). A compact oxide layer is formed at the beginning of the electrolysis (Fig. 20.3). Simultaneously, the current decreases, due to the electric resistance of the oxide. Subsequently follows a process in which the oxide is redissolved by the acid, and the current increases until it reaches a steady state. The electrochemical oxidation continues to take place with formation of pores. At the end of a pore, where it has the largest curvature, the electric field has its largest gradient and the process of redisolution is fastest. 

Fichter and Kern O first reported that uric acid could be electrochemically oxidized. The reaction was studied at a lead oxide electrode but without control of the anode potential. Under such uncontrolled conditions these workers found that in lithium carbonate solution at 40-60 °C a yield of approximately 70% of allantoin was obtained. In sulfuric acid solution a 63% yield of urea was obtained. A complete material balance was not obtained nor were any mechanistic details developed. In 1962 Smith and Elving 2) reported that uric acid gave a voltammetric oxidation peak at a wax-impregnated spectroscopic graphite electrode. Subsequently, Struck and Elving 3> examined the products of this oxidation and reported that in 1 M HOAc complete electrochemical oxidation required about 2.2 electrons per molecule of uric acid. The products formed were 0.25 mole C02,0.25 mole of allantoin or an allantoin precursor, 0.75 mole of urea, 0.3 mole of parabanic acid and 0.30 mole of alloxan per mole of uric acid oxidized. On the basis of these products a scheme was developed whereby uric acid (I, Fig. 1) is oxidized in a primary 2e process to a shortlived dicarbonium ion (Ha, lib, Fig. 1) which, being unstable, under-... 

Metal dithiophosphinato complexes are usually prepared by metathesis of metal halides with alkali metal or ammonium dithiophoshinates, but can also be conveniently prepared by reactions of /i .v(thiophosphinyl)disul fanes, R2(S)PSSP(S)R2, with metal species.87 The electrochemical oxidation of metals in acetonitrile solution, in the presence of diphenylphosphine and sulfur affords M(S2PPh2)2 (M = Co, Zn, Cd),88 but this is not a preparative method. 

Smooth polycrystalline Au, Pt and Ir thin-layer electrodes were utilized (10-11). Electrodes were cleaned between trials by sequential electrochemical oxidation above 1.2 V [Ag/AgCl (1 M Cl-) reference] and reduction below -0.2 V in 1 M H2SO4 surface cleanliness was verified with the aid of cyclic voltammetry in the same molar sulfuric acid solution. Experiments were carried out in 1 M H2SO4, 1 M NaC104 buffered at pH 7 and 10, and in 1 M NaOH solutions were prepared with pyrolytically triply distilled water (12). Surface reagents employed were iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol [DHT (13)1. and 3,6-dihydroxypyridazine (DHPz). 

Direct production of benzoquinone (BQ) from benzene is one of the targets in industrial chemistry. Considerable efforts have been made to develop the electrochemical oxidation of benzene to p-benzoquinone to the industrial scale thus forming a basis for a new hydroquinone process [40]. Benzene in aqueous emulsions containing sulfuric acid (1 1 mixture of benzene and 10% aqueous H2S04) forms, at the anode, p-benzoquinone which can be reduced cathodically to yield hydroquinone in a paired synthesis. A divided cell with Pb02 anodes is used. 

Only the inorganic polysulfides (and more specifically the alkali polysulfides) will be considered. Furthermore, we shall not review the electrochemical properties of SxOy ions or molecules. It must be noted that a comprehensive review, devoted to the oxidation of sulfur(IV) oxides, oriented toward the atmospherically relevant processes and mechanisms, appeared in 1995... 

Other sulfur-containing compounds Pardo et al. [115] have studied electrochemical oxidation of 2-mercaptopyridine N-oxide on Hg electrodes in aqueous solutions. It has been found that anionic form of this compound was oxidized to yield a radical. At low concentration, oxidation was reversible and the radical was strongly adsorbed. At high concentrations, this process was accompanied by diffusive reversible oxidation. Reorientation of the adsorbed 2-mercaptopyridine N-oxide molecules on Hg electrodes in the presence of Triton X-100 has been studied... 

The first structural report on a phthalocyanine complex concerned [Ni(pc)J (Table 110 I).2878 In the crystal lattice of this compound the square planar macrocycles are arrayed in slipped stacks such that the distance between the molecular planes along the perpendicular direction is 388 pm. [Ni(pc)] may be prepared by a variety of methods 2873,2871 2880 a convenient one is heating a foil of elemental nickel in o-cyanobenzamide at 270 °C (Scheme 60).2881 [Ni(pc)] is insoluble in the most common organic solvents, but soluble in concentrated sulfuric acid from which it is reprecipitated unchanged upon dilution. This complex is thermally very stable and may be sublimed in vacuo. The reduction of [Ni(pc)] can be accomplished by chemical or electrochemical methods and results in ligand-based reduced anions [Ni(pc)]" ( = 1, 2). Analogously, the electrochemical oxidation results in the oxidized ligand. 3... 

Nicotinic acid and nicotinamide, members of the vitamin B group and used as additives for flour and bread enrichment, and as animal feed additive among other applications, are made to the extent of 24 million pounds (nearly 11 million kilograms) per year throughout the world. Nicotinic acid (pyridine-3-caiboxylic acid), also called niacin, has many uses. See also Niacin. Nicotinic acid is made by the oxidation of 3-picolme or 2-mcthyl-5-cthylpyridine (the isocinchomcnc acid produced is partially deearboxylated). Alternatively, quinoline (the intermediate quinolinic acid) is partially deearboxylated with sulfuric add in the presence of selenium dioxide at about 300° C or with nitric acid, or by electrochemical oxidation. Nicotinic acid also can be made from 3-picoline by catalytic ammoxidation to 3-cyanopyridine, followed by hydrolysis. 

A library of 35 different catalysts fixed on electrochemically oxidized aluminum either in oxalic acid (Lib 1) or sulfuric add (lib 2) was tested at 450 °C and 1.1 bar. The methane-to-oxygen ratio was set to 1 in order to establish the potential of the catalyst to form intermediates. Figure 3.20 shows experimental results for a residence time of 550 ms and a screening time of 60 s. The conversion rate followed directly the platinum content in the catalysts. The higher the platinum content, the higher is the degree of conversion. Catalyst carrier formed by anodization of... 

The substituted tricarbonyl complexes react in a similar fashion to the tetracarbonyl complexes with halogens, whereas electrochemical oxidation appears to cause the formation of a doubly charged cation, [M(CO)3 (triars)]2+, where M = Cr, Mo, W (67). Allyl halides react with the tripyridine-substituted carbonyls of the type Mo(CO)3(bipy)py [or Mo(CO)3 (phen)py phen = phenanthroline] to form a 77-allylic carbonyl cation, such as [Mo(CO),(bipy)py(7r-allyl)]+ (146). The cation reacts with sulfur dioxide to form Mo(CO)2(bipy)(S02)2 (147). 

Radical cations that are produced by electrochemical oxidation are not stable in solvents with appreciable base character. This results because such radicals are subject to attack by available nucleophiles, and solvents that contain donor electron pairs are good nucleophiles. Cation radicals are most stable in solvents that are good Lewis acids and show negligible basic properties. Some of the solvent systems that have been employed to stabilize electrochemically produced cation radicals include nitromethane and nitrobenzene,21 dichloro-methane,22 trifluoroacetic acid-dichloromethane (1 9),23 nitromethane-AlCl3,24 and AlCl3-NaCl (1 l).25 Organic chemists should be familiar with the stabilization of carbonium ions by superacid media.26 These media usually contain fluorosulfuric acid, or mixtures of fluorosulfuric acid with antimony pen-tachloride and sulfur dioxide, and are potent solvents for the production and stabilization of organic cations. 

It has been proposed that mediated electrochemical oxidation may be used for the ambient temperature destruction of hazardous waste. Using Co(III)- -mediated electrochemical oxidation in sulfuric acid, l,3-dichloro-2-propanol and 2-chloro-l-propanol were oxidized to carbon dioxide259. A series of studies is being conducted on the use of the anodic oxidation of barium peroxide to produce an intermediate that leads to destruction of halogenated organic compounds. 

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