Alcohol electrocatalytic reduction

However, for some electrocatalytic reactions, such as the electrooxidation of alcohols, aldehydes or acides, and also the electro reduction of oxygen, lead adatoms can exhibit a promoting effect (3-7). Moreover, lead can change the selectivity in the case of electrocatalytic reductions of nitrocompounds (8), whereas it inhibits the adsorption of hydrogen on platinum (9,10),... 

The features of the electrode used in this gas-phase electrocatalytic reduction of C02 are close to those used in PEM fuel cells [37, 40, 41] (e.g. a carbon cloth/Pt or Fe on carbon black/Nafion assembled electrode, GDE). The electrocatalysts are Pt or Fe nanoparticles supported on nanocarbon (doped carbon nanotubes), which is then deposited on a conductive carbon cloth to allow the electrical contact and the diffusion of gas phase C02 to the electrocatalyst. The metal nanoparticles are at the contact of Nation, through which protons diffuse. On the metal nanoparticles, the gas-phase C02 reacts with the electrons and protons to be reduced to longer-chain hydrocarbons and alcohols, the relative distributions of which depend on the reaction temperature and type of metal nanoparticles. Isopropanol forms selectively from the electrocatalytic reduction of C02 using a gas diffusion electrode based on an Fe/N carbon nanotube (Fe/N-CNT) [14, 39, 40]. Not only the nature of carbon is relevant, but also the presence of nanocavities, which could favor the consecutive conversion of intermediates with formation of C-C bonds. 

Aliphatic carboxylic acids are reducible to the alcohol in low yield in strongly acidic electrolytes. Thus, phenylacetic acid is reduced to 2-phenylethanol in 25-50% yield at a lead cathode in 50% sulfuric acid-ethanol [39] and butyric acid to butanol in 80% sulfuric acid in 6.5% yield [40]. The latter conversion can also be performed in 17% yield in 7% aqueous sodium hydroxide, despite the fact that the carboxylate form of the acid, which is reducible only with difficulty, predominates in bulk solution, suggesting an electrocatalytic reduction involving specific interaction of the substrate and electrode surface. 

The different electrocatalytic reactions described below concern the oxidation of molecular hydrogen and of small organic molecules and alcohols, the reduction of protons and of dioxygen, the electrohydrogenation of organic molecules, the reduction of halides and the electroreduction of carbon dioxide. 

Ogura, K. and K. Takamagari (1986). Electrocatalytic reduction of carbon dioxide to methanol. Part 2. Effects of metal complex and primary alcohol. J. Chem. Soc. Dalton Trans. 10, 1519-1523. 

Other interesting examples of electrocatalysis include the selective transformation of benzyl alcohol to benzaldehyde by RuO particles incorporated in polypyrrole-bipyridyl complexes and electrocatalytic reduction of O2 by polypyr-role-cobalt(II) Schiff s base complexes. 

Electrocatalytic regeneration of NADH has also been performed at polymer-modified electrodes with pendent [(r/s-Cs M cs) R h( bpy )C1]1 55,56 and [Rh(terpy)2]3+ 57 complexes, and coupled to enzymatic reduction with alcohol dehydrogenase.55,57... 

Cathodic surfaces of finely divided platinum, palladium and nickel have a low hydrogen overvoltage and the dominant electrochemical reaction is the generation of a layer of hydrogen atoms. The electrocatalytic hydrogenation of aldehydes and ketones can be achieved at these surfaces. Cathodes of platinum or palladium black operate in both acid solution [203] and in methanol containing sodium methoxide [204], The carbonyl compound is converted to the alcohol. Reduction of 4-tert-butylcyclohexanone is not stereoselective, however, 1,2-diphenylpropan-l-one is converted to the / reo-alcohol. 

There are certain combinations of electrode materials, solvents and operating conditions, which allow the reduction of C02 to afford hydrocarbons. It was concluded in a comparative study using many different metal electrodes in aqueous KHCO3 solution that either CO (Ag, Au) or formic acid (In, Sn, Hg, Pb) are produced as a result of the reduction of C02, and Cu has the highest electrocatalytic activity for the production of hydrocarbons, alcohols, and other valuable products.137 Most of the studies, since the mid-1990s, consequently, have focused on the further elucidation of electrocatalytic properties of copper. 

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. 

A number of important synthetic processes involving mediated reactions of halides rather than direct electrochemical reduction have been reported. The reduction of aryl halides in the presence of alcohols with or without NH3 as a cosolvent leads to the oxidation of the alcohols to the carbonyl compounds. The reaction involves an electrocatalytic process mediated by electron transfer from the initially reduced aryl halide. Alcohols such as benzhydrol and 2-propanol are converted to their respective ketones, on a preparative scale163. The proposed mechanism is shown in Schemes 14 and 15. 

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 behavior of cathodes appears to play a crucial role in the reduction of carbon dioxide. To find more efficient catalysts, detailed mechanistic studies of CO2 reduction are needed. Further studies could also concentrate on the investigation of different electrolytes as well as different catalysts delivering products of choice, such as alcohols. 

A sputtered platinum counter-electrode on a TCO substrate may have its electrocatalytic activity, in the reduction of the tri-iodide ions, enhanced by creating Pt colloids from an alcoholic solution of H2PtCl6. 

Metal electrodes are divided into 4 groups in accordance with the product selectivity indicated in Table 3. Pb, Hg, In. Sn, Cd, Tl, and Bi give formate ion as the major product. Au. Ag, Zn. Pd, and Ga, the 2nd group metals, form CO as the major product. Cu electrode produces CH4, C2H4 and alcohols in quantitatively reproducible amounts. The 4th metals, Ni, Fe, Pt, and Ti. do not practically give product from CO2 reduction continuously, but hydrogen evolution occurs. The classification of metals appears loosely related with that in the periodic table. However, the correlation is not very strong, and the classification such as d metals and sp metals does not appear relevant. More details of the electrocatalytic properties of individual metal electrodes will be discussed later. 

The work presented shows that an increase of the electrocatalytic activity can be obtained, if a suitable method for the catalyst synthesis is employed. In this sense, the Alcohol Reduction Method showed a positive effect, probably due to the good particle dispersion at the carbon surface and the suitable particle size distribution that this method produces. For the methanol oxidation results, an increase in the cell potential by PtRu/C electrocatalyst on Vulcan XC72 system was observed compared to the PtRu/C E-TEK formulation. This can be explained due to the better conductivity of this Carbon Suport, enhancing the speed of the electron transference in the Methanol Oxidation Reaction (MOR).These results can also be attributed to the good particle distribution at... 

On the basis of these results, general schemes of electrocatalytic behavior of the molecules were suggested involving oxidation and reduction processes with the participation of adsorbed species. The schemes suggested for n-propanol (PrOH) and allyl-alcohol (AA) are as follows ... 

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