Direct electrochemical oxidation

Several activities, if successful, would strongly boost the prospects for fuel ceU technology. These include the development of (/) an active electrocatalyst for the direct electrochemical oxidation of methanol (2) improved electrocatalysts for oxygen reduction and (2) a more CO-tolerant electrocatalyst for hydrogen. A comprehensive assessment of the research needs for advancing fuel ceU technologies, conducted in the 1980s, is available (22). 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

The direct electrochemical oxidation of manganese alloys was developed and commercialized at the Rustavi Chemical Combine in the Georgian Repubhc (formerly the USSR). The electrode reactions are... 

Gorte RJ, Kim H, and Vohs JM. Novel SOFC anodes for the direct electrochemical oxidation of hydrocarbon. J. Power Sources 2002 106 10-15. 

Because the direct electrochemical oxidation of NAD(P)H has to take place at an anode potential of + 900 mV vs NHE or more, only rather oxidation-stable substrates can be transformed without loss of selectivity—thus limiting the applicability of this method. The electron transfer between NADH and the anode may be accellerated by the use of a mediator. At the same time, electrode fouling which is often observed in the anodic oxidation of NADH can be prevented. Synthetic applications have been described for the oxidation of 2-hexene-l-ol and 2-butanol to 2-hexenal and 2-butanone catalyzed by yeast alcohol dehydrogenase (YADH) and the alcohol dehydrogenase from Thermoanaerobium brockii (TBADH) repectively with indirect electrochemical... 

Direct electrochemical oxidation is not a convenient way for a preparative production of carbonyl compounds from alcohols due to the unselectivity caused by the high oxidation potentials of alcohols. Thus, there have been only a few compounds (some aliphatic alcohols, glycols, and related alcohols) that have been oxidized by the direct method, while the indirect method has often been used to oxidize selectively a variety of alcohols, since it does not... 

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. 

The direct electrochemical oxidation of alcohols is in many cases unselective because of the high oxidation potentials of the alcohols. One possible way to avoid this disadvantage is the use of a mediatory system (an indirect oxidation). Thereby a mediator is converted to its oxidized form at a less positive potential than that required for the direct oxidation of the alcohol, and the oxidized form of the... 

There has been some controversy in the literature over precisely what should be called direct oxidation or direct utilization of hydrocarbons in an SOFC. As pointed out by Marina and Mogensen and Park et al., direct, electrochemical oxidation of complex hydrocarbons is unlikely to occur in one step. Even in the case of methane, the reaction produces eight electrons and must almost certainly occur in multiple steps. 

Vinylindote radical-cations, for example that derived from 64, take part in a Diels-Alder reaction with alkenes. Subsequent oxidation of the initial product with loss of two protons and dimethylamine gives the pyrido[l,2a]indoie. Reaction is achieved either by direct electrochemical oxidation or by photochemical electron... 

The direct electrochemical oxidation of alcohols involves removal of one electron from a non-bonding pair on oxygen. Relatively anodic potentials are required and the use of reagents, which can provide another mechanism for the oxidation step, has been extensively explored. Electrochemistry is then involved in the reoxidation of spent reagent and often the system can be adapted so as to require only a catalytic amount of reagent. 

Direct electrochemical oxidation of protected a-amino acids is generally ineffective. An exception is provided by proline derivatives, which are methoxylated on carbon-5 of the pyrrolidine ring. Open chain protected a-amino acids undergo... 

The direct electrochemical oxidation of phenols generates phenoxonium cations which are able to undergo [3-1-2] cycloaddition in the presence of unactivated alkenes to produce benzofurans <1999JOC7654>. Thus, electrolysis of methyl 2,5-dihydroxybenzoate in a solution of lithium perchlorate in nitromethane in the presence of acetic acid and 2-methyl-2-butene produces the dihydrobenzofuran in excellent yield (Equation 88). 

It is generally agreed that the mechanism describing this process can be explained by a sequence of direct electrochemical oxidations of the organic substrate, followed by addition/elimination reactions of fluoride and hydrogen ions,... 

The proposed mechanism is illustrated in Scheme 3.2. It is important to realize that the mechanism does not take into account the direct electrochemical oxidation of the inclusion complex. Evidence for this surprising finding was gathered at fast scan rates, at which the cyclic voltammetric wave for oxidation of FcCOO- is flattened,19 because the complex dissociation mechanism becomes too slow to generate enough free guest to sustain the fast electrochemical oxidation. 

The direct electrochemical oxidation of aliphatic alcohols occurs at potentials which are much more positive than 2.0 V w. SCE. Therefore, the indirect electrolysis plays a very important role in this case. Using KI or NaBr as redox catalysts those oxidations can be performed already at 0.6 V vs. SCE. Primary alcohols are transformed to esters while secondary alcohols yield ketones In the case of KI, the iodo cation is supposed to be the active species. Using the polymer bound mediator poly-4-vinyl-pyridine hydrobromide, it is possible to oxidize secondary hydroxyl groups selectively in the presence of primary ones (Table 4, No. 40) The double mediator system RuOJCU, already mentioned above (Eq. (29)), can also be used effectively Another double mediator system... 

Cysteine Direct electrochemical oxidation at bare SPCEs Amperometry in stirred solution... 

Uric acid in blood Direct electrochemical oxidation at bare SPCE swv Scan from —0.2 to + 1.0V 200-1000 pM <300pM Chen et al. [131]... 

Direct Electrochemical Oxidation of Propylene in a Sparged Packed-Bed Electrode Reactor19,20... 

The direct electrochemical oxidation (no cell divider membrane) of wastewater has been employed in the textile industry. Typically, this industry produces an organic-contaminated wastewater that also contains sodium chloride sodium chloride is desirable in promoting anodic oxidation. The presence of sodium chloride is fortuitous for textile manufacturers since the hypochlorite byproduct produced in the electrochemical oxidation process is used for textile bleaching operations.24... 

Murphy OJ, Hitchens GD, Kaba L, Verostko CE. Direct electrochemical oxidation of organics for wastewater treatment. Water Res 1992 26 443-451. 

---