Electro-oxidation of Hydrocarbons

Most of the research involving the electrochemical conversion of hydrocarbons has been investigated primarily with platinum either as platinized metal or dispersed on a carbon support matrix. Early work focused upon the oxidation of the lower aliphatic hydrocarbons and partially oxygenated derivatives. Below is a table illustrating the relative activities of basic fuels investigated in basic media  

Fuel Current density (Aft- ) Electrical voltage vs. theoretical O2 electrodes 

From the initial studies, the following trend of reactivity was observed generally in order of decreasing activity monohydric alcohols olefins tertiary alcohols 

There is no doubt that total oxidation of hydrocarbons can occur at certain electrode surfaces in acidic media. ° Early research by Schlatter on the electrochemical oxidation of saturated and unsaturated hydrocarbons (such as propane and ethylene) in a 5 N H2SO4 electrolyte was found to result in complete or almost complete oxidation to CO2 and H2O. These tests were made in a two electrode cell equipped with an anion exchange membrane which served to separate the catholyte and anolyte. The oxidation half-reactions of a number of aliphatic hydrocarbons, including methane, ethane, and propane, being converted directly to carbon dioxide is listed below. 

Although the reversible potentials of these fuels are very close to that of hydrogen, the polarization of electrodes supplied with gaseous hydrocarbons is much greater than with hydrogen. The performance of ethane, propane, and 

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The electro-oxidation of hydrocarbons fuel cells operating on such fuels... 

The earliest intensive research into the direct electro-oxidation of hydrocarbons for fuel cell applications was in the late 1950 s and the 1960 s. Early attempts to anodically oxidize hydrocarbons were disappointing, in that low rates were observed with both the lower molecular weight gases and higher molecular weight hydrocarbons. To achieve acceptable rates of oxidation. 

To conclude, a large of amount of research has been performed, most notably in the 1960 s, with the intent of developing electrocatalysts and electrochemical fuel cell systems which use hydrocarbon-based fuels. However, due to the limited success achieved to-date in the electro-oxidation of hydrocarbons, more recently, effort has focused upon the use of hydrogen as... 

Flg. 1.114 Three electrode cell design used for the electro-oxidation of hydrocarbons and the electro-reduction of oxygen using gas-diffusion-type electrodes. 

Due to the intense efforts to develop fuel cell technology in the 1960 s, there has been continued interest in identifying efficient electrocatalytic processes for the electro-oxidation of organic molecules. The prospect of achieving the conversion of the chemical energy of hydrocarbons and their... 

The oxidation of hydrogen was also studied on R gas-diffusion electrodes and compared with the results obtained from the investigation of hydrocarbons. As expected, the results show that electro-oxidation of hydrogen occurs much more readily than that of propane or methane on R. When a Nafion coated electrode is used instead of bare R, high current densities can be obtained with little polarization, as shown in Fig. 1.8. 

The fundamental studies of the electro-oxidation of low molecular weight, saturated hydrocarbons in aqueous electrolytes such as sulfuric and phosphoric acid, showed that adsorption of the organic molecule upon the surface of the catalytic electrode is a critical Partial oxidation occurs rapidly,... 

As with the electro-oxidation of aliphatic hydrocarbons, several pathways may be involved in anodic reactions of aromatic molecules (a) direct electron transfer from the aromatic substrate to form cationic species, (b) reaction of... 

In solution of perchlorates and tosylates in acetic acid, the primary products of electro-oxidation of aromatic hydrocarbons are shown to be benzylic acetates, i.e., substitution occurs preferentially in the side chain. A large, primary isotope effect, ku/kjy = 2.6, provided strong evidence that in this type of substitution reaction the rate-determining step involved the loss of a proton.Further refinement of the mechanism correlated isomeric product ratios to the distribution of positive charge density in the intermediate cation radical. 

The electro-oxidation of an aromatic hydrocarbon containing a benzylic hydrogen, using solutions of perchlorates, tetrafluoroborates, or hexafluorophosphates in acetonitrile as the electrolyte, results in the formation of an acetamido derivative. This is essentially the electrochemical analog of the Ritter reaction. Again direct oxidation of the aromatic molecule has been regarded as the initial step in the overall reaction, e.g., in the reaction with toluene (II) (Scheme 2). Increasing amounts of the nucleophile, water. 

A significantly different viewpoint has been expressed wherein it is suggested that the formation of an oxide layer at the platinum anode is necessary for the formation of the hydrocarbon. The presence of a layer of platinum oxide at high anodic potentials appears to be supported by data obtained from a study of the platinum electrode surface using ESCA. t On the other hand, polaromicrotribometry —the measurement of the coefficient of friction of a metal surface—showed that in the electro-oxidation of acetate ions an organic film is formed in that region just below the critical potential. 

Here, M represents the electronically conducting electrode material (e.g.. Ft) that is not involved in the overall reaction and plays the role of an electrocatalyst for the reaction. The last intermediate step occurs in two identical consecutive steps since electron transfer occurs by quantum mechanical tunneling, which involves only one electron transfer at a time. When multistep reactions take place, there is the possibility of parallel-intermediate steps. The parallel-step reactions could lead to the same final product or to different products. Direct electro-oxidation of organic fuels, such as hydrocarbons or alcohols, in a fuel cell exhibits this behavior. For instance, in the case of methanol, a six-electron transfer, complete oxidation to carbon dioxide can occur consecutively in six or more consecutive steps. In addition, partially oxidized reaction products could arise, producing formaldehyde and formic acid in parallel reactions. These, in turn, could then be oxidized to methanol. 

Electro-organic chemistry is the study of the oxidation and reduction of organic molecules and ions, dissolved in a suitable solvent, at an anode and cathode respectively in an electrolysis cell, and the subsequent reactions of the species so formed. The first experiment of this type was reported in 1849 by Kolbe, who described the electrolysis of an aqueous solution of a carboxylate salt and the isolation of a hydrocarbon. The initial step involves an anodic oxidation of the carboxylate anion to a radical which then dimerises to the alkane. 

The improved electro-oxidation behavior observed with the C-8 and C-12 acid coated electrodes might be attributed to (i) the higher concentration of sulfonic acid groups present in the electrocatalytic layer of the C-8 acid and C-12 acid coated electrode, (ii) enhanced diffusion of reactants and products, and/or (iii) the enhanced wettability" at the catalytic site of oxidation facilitating hydrocarbon adsorbtion. These results are in accordance with earlier findings where the addition of C-8 acid was observed to significantly enhance the ease of oxidation of various oxygenated molecules. 

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