Electrocatalytic Oxidation of Alcohols

With the development of fuel cells, electrocatalytic oxidation of small organic molecules, such as methanol or formic acid, has attracted great interest recently (Rice et al., 2003). Ethanol oxidation to acetaldehyde can be performed by means of the reactions  

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By using various polypyridyl oxo complexes a relationship between redox potentials ( 1/2) of the complexes and the efficiency and the selectivity of the electrocatalytic oxidation of alcohols and diols has been established.506 Higher 1/2 gives higher reactivity. The best results, from the point of view of synthesis, were obtained with the complex /ra ,v-[Ru (terpy)(0)2(0I I2)]2 which is characterized by a high redox potential and a relatively high stability. 

Until recently there has been surprisingly little interest in high oxidation state complexes of terpy. Meyer and co-workers have demonstrated that the ruthenium(IV) complex [Ru(terpyXbipy)0] is an effective active catalyst for the electrocatalytic oxidation of alcohols, aromatic hydrocarbons, or olefins (335,443,445,446). The redox chemistry of the [M(terpy)(bipy)0] (M = Ru or Os) systems has been studied in some detail, and related to the electrocatalytic activity (437,445,446). The complexes are prepared by oxidation of [M(terpy)(bipyXOH2)] . The related osmium(VI) complex [Os(terpyXO)2(OH)] exhibits a three-electron reduction to [Os(terpyXOH2)3] (365,366). The complex [Ru(terpy)(bipyXH2NCHMe2)] undergoes two sequential two-electron... 

The electrocatalytic oxidation of alcohols is possible by using modified conductive polymer electrodes. One of the most interesting examples of such a reaction is the electro-oxidation of methanol with highly dispersed platinum-based particles inserted in a polymeric matrix. 

Kwon Y, Lai SCS, Rodriguez P, Koper MTM (2011) Electrocatalytic oxidation of alcohols on gold in alkaline media base or gold catalysis J Am Chem Soc 133 6914-6917... 

Ureta-Zanartul MS, Berrios C, Gonzalez T, Fernandez F, Baez D, Salazar R, Gutierrez C (2012) Electrocatalytic oxidation of alcohols at gold electrodes in alkaline media Int J Electrochem Sci 7 8905-8928... 

Ciszewski, A. and G. Milczarek (1996). Electrocatalytic oxidation of alcohols on glassy carbon electrodes electrochemically modified by conductive polymeric nickel(II)tetrakis(3-methoxy-4-hydroxyphenyl)porphyrin film. J. Electroanal. Chem. 413, 137-142. 

In this chapter, the authors review some developments in the application of Pd-based nanostructures for the electrocatalytic oxidation of alcohols in alkaline media. Special focus is given to the role of such nanostructures (mono-, hi-, or ternary metallic catalysts) and/or their supporting platforms in lowering the anodic overpotential (onset potential), enhancing the catal3dic current density, and improving the stability or lifetime of the catalysts. 

Roslonek,G, and Taraszewska,J. "Electrocatalytic oxidation of alcohols on glassy carbon electrodes electroche-mically modified with nickel tetraazamacrocyclic complexes. Mechanism of film formation", J.Electroanal.Chem., (in press). 

The organic compounds were dissolved or dispersed in an aqueous solution of the catalyst, with or without an organic cosolvent, and the net oxidations were carried out at applied potentials causing the oxidation of Ru11 to RuIV complex (0.6-0.8 V vs. SCE). It has been demonstrated that this electrocatalytic system is capable of providing a general and selective method for the oxidation of alcohols, aldehydes, cyclic ketones, and C—11 bonds adjacent to alkeneic or aromatic groups. 

Techniques for attaching such ruthenium electrocatalysts to the electrode surface, and thereby realizing some of the advantages of the modified electrode devices, have been developed.512-521 The electrocatalytic activity of these films have been evaluated and some preparative scale experiments performed. The modified electrodes are active and selective catalysts for oxidation of alcohols.5 6-521 However, the kinetics of the catalysis is markedly slower with films compared to bulk solution. This is a consequence of the slowness of the access to highest oxidation states of the complex and of the chemical reactions coupled with the electron transfer in films. In compensation, the stability of catalysts is dramatically improved in films, especially with complexes sensitive to bpy ligand loss like [Ru(bpy)2(0)2]2 + 51, 519 521... 

C. Lamy, E. M. Belgsir, and J.-M. Leger, Electrocatalytic oxidation of aliphatic alcohols Application to the direct alcohol fuel cell (DAFC), J. Appl. Electrochem. 31, 799-809 (2001). 

As shown by their redox potentials oxoruthenium(IV) species containing polypyridyl ligands are strong oxidants and they oxidize a variety of substrates. The complex [Ru(0)(bpy)2(py)] has also been used electrocatalytically for the oxidation of alcohols, aldehydes, alkenes, and aromatics." Electrocatalytic oxidation has also been performed on this complex that has been incorporated into poly-4-vinylpyridine. ... 

The electrocatalytic oxidation of several secondary and primary alcohols has been also described, in keeping with the original work by Masui and coworkers it resorts to HPI or to X-substituted HPIs as electron carriers . The tetrafluoroaryl-substituted HPI was the most efficient among these catalysts. Secondary alcohols gave carbonyl compounds primary alcohols gave the corresponding aldehyde exclusively under anaerobic conditions, whereas a mixture of aldehyde plus carboxylic acid was formed in the presence of 2 . 

Oxidation of Alcohols in a Direct Alcohol Fuel Cell The electrocatalytic oxidation of an alcohol (methanol, ethanol, etc.) in a direct alcohol fuel cell (DAFC) will avoid the presence of a heavy and bulky reformer, which is particularly convenient for applications to transportation and portable electronics. However, the reaction mechanism of alcohol oxidation is much more complicated, involving multi-electron transfer with many steps and reaction intermediates. As an example, the complete oxidation of methanol to carbon dioxide ... 

Scheme 12 Electrocatalytic oxidation of benzylic alcohol to benzylic aldehyde using [Cr" (0H2)PWii039] as redox catalyst taken from Ref 8).

An alternative biosensor system has been developed by Hart et al. [44] which involves the use of the NAD+-dependent GDH enzyme. The first step of the reaction scheme involves the enzymatic reduction of NAD+ to NADH, which is bought about by the action of GDH on glucose. The analytical signal arises from the electrocatalytic oxidation of NADH back to NAD+ in the presence of the electrocatalyst Meldola s Blue (MB), at a potential of only 0Y. Biosensors utilising this mediator have been reviewed elsewhere [1,17]. Razumiene et al. [45] employed a similar system using both GDH and alcohol dehydrogenase with the cofactor pyrroloquinoline quinone (PQQ), the oxidation of which was mediated by a ferrocene derivative. 

Similar reaction pathways have also been found for the oxidation of dimethyl sulfide to dimethyl sulfoxide and dimethyl sulfoxide to dimethyl sulfone by [Ru(bpy)2(py)(0)]2+ with respective rate constants of 17.1 and 0.13 M l s"1 in MeCN at 298 K (48). The complex [Ru(bpy)2 (py)(0)]2+ has also been used electrocatalytically for the oxidation of alcohols, aldehydes, alkenes, and aromatics (23, 49). The kinetics of oxidation of formic acid/formate ion by [Ru(bpy)2(py)(0)]2 +, with a large kinetic isotope effect [ HC02-/ADCo2- = 19 (25°C, /r = 1.0 M)], has been reported (50). A two-electron hydride transfer has been suggested for the oxidation of HC02 by [Ru(bpy)2(py)(0)]2+. A similar mechanism has also been suggested for the oxidation of alcohols (51) and aromatics (52) by [Ru(bpy)2(py)(0)]2+ and other related Ru(IV) oxo complexes (28,... 

Ascorbic acid (1) is most commonly used for testing the performance of electrodes in redox systems. Thus, a Ag-Ag ascorbate selective electrode was constructed with view to use it for vitamin C determination. Its reproducibility and stability was satisfactory and ascorbate ion concentration could be determined in neutral, alkaline and alcoholic media" . A voltametric study was carried out for the evaluation of graphite-epoxy composite (GEC) electrodes for use in the determination of ascorbic acid and hydroquinone. They were compared with mercury and CPE in similar operating conditions of pH and supporting electrolytes. Like all redox electrodes, also GEC electrodes deteriorate on exposure to air or after repeated usage, and the surface had to be renewed for activation. GEC electrodes were found to be adequate for redox system analyses"". The electrocatalytic oxidation of 1 is an amplification method for determination of specific miRNA strands using the An biosensor described in Table 1 . 

The electrocatalytic oxidation of the primary alcohol (in C6 position) is similar to the reaction mechanism that is well known for methanol. Therefore, the transformation of gluconic acid into glucaric acid can be written as follows ... 

Chi Q. and Dong S., Electrocatalytic oxidation of reduced nicotinamide coenzymes at methylene green-modified electrodes and fabrication of amperometric alcohol biosensors, Anal. Chim. Acta., 285, 125-133, 1994. 

The development and application of carbonate as well as anion-exchange PEM electrolytes have significantly renewed interest in the development of alkaline-based DAFC [134]. Many of the reaction intermediates, products and paths discussed above for the catalytic oxidation of alcohols in alkaline media have also been identified or speculated to take part in the electrocatalytic oxidatiOTi of these same alcohols. 

As specified above, it has also been shown that tetraaza[14]annulene 74 (5,7,12,14-tetramethyl-dibenzo-l,4,8,ll-tetraaza[14]annulene) can be electrode-posited on common conducting substrates and electrochemically treated so as to display a nickel-based catalytic deposit towards the oxidation of carbohydrates in alkaline solutions which shows strong similarities to the nickel hydroxide electrode. Similar observations were reported for Ni complex 75 although they were rather oriented towards the oxidation of hydrogen peroxide " and for Ni curcumin films towards the electrocatalytic oxidation of aliphatic alcohols. ... 

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