Electrochemical oxidation of hydrocarbons

A period of high research activity in electrocatalysis began after it had been shown in 1963 that fundamentally, an electrochemical oxidation of hydrocarbon fuel can be realized at temperatures below 150°C. This work produced a number of important advances. They include the discovery of synergistic effects in platinum-ruthenium catalysts used for the electrochemical oxidation of methanol. 

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

The purpose of the present chapter is to summarize the electrochemical oxidation of hydrocarbons with special reference to the electrode processes involved. The reader may also find material of interest in Chapters 22 (Electrolytic oxidative coupling), 24 (Anodic substitution and addition), and 32 (Conducting polymers). 

The effects of molecular structure, electrolyte and temperature on the rate and extent of adsorption and electrochemical oxidation of hydrocarbons (alkanes, alkenes, alkynes) have been reviewed . ... 

An important application of phosphoric acid is in fuel cells (PAFC) where it serves as an electrolyte in the electrochemical oxidation of hydrocarbons at about 250 C. Such cells, using 100% H3PO4 in... 

The study of electrochemical oxidation of hydrocarbons, except for a few compounds like benzene, was begun only very recently (primarily beginning about 1960) and as yet little has been published in this area. Several studies have been made of the performance of various hydrocarbons in fuel cells with determination of current efficiency and product analysis (cf. introduction) however, kinetic parameters were not determined. The few systems which have been studied from a proper mechanistic point of view will be discussed here. 

To a certain extent, the decline of interest in fuel cells was due to difficulties in the conunercial realization of earlier achievements. Despite the demonstration that low-temperature electrochemical oxidation of hydrocarbons is basically possible, reaction rates realized in practice were too low, and the amounts of platinum-metal catalyst required to achieve them were so large that the economic prospects of fuel cells using these reactions were very poor. Platinum catalysts were used in most of the fuel cell built, despite the fact that in many studies it had been shown that nonplatinum catalysts could be useful for hydrogen and oxygen electrodes. For economic reasons, the number of potentially interested users decreased gradually. The financial support of work on fuel cells decreased correspondingly. 

For the measurement of gas components like hydrocarbons (HC) or nitric oxides (NOx) in non-equilibrated gas phases kinetically determined sensors are used (Fig. 19.2 middle).Depending on the electrode material, the gas components do not equilibrate on the measuring electrode at temperatures <700 °C. Thus gas components which are not thermodynamically stable are electrochemically active. In an HC- and 02-containing gas, for example, at least two electrode reactions can take place the electrochemical reduction of oxygen and the electrochemical oxidation of hydrocarbons. The measured open-circuit voltage does not obey the Nemst equation. Therefore such electrode behaviour is often referred to non-Nernstian electrodes (or mixed potential sensors). The cell voltage depends logarithmically on the concentrations of the hydrocarbons ... 

Binder and others [230] found that Raney platinum was the best single catalyst for electrochemical oxidation of hydrocarbons. Data obtained by Bianchi [231], Grubb [232], and Cairns and Mclnemey [203], who compared the catalytic activities of various platinum blacks in the anodic oxidation of propane, are consistent with this conclusion. As found by Petrii and Marvet [100], the oxidation rate of methane decreases sharply on changing from platinum to other platinum catalysts. Bockris and Dahms [192] showed that the oxidation rate of ethylene decreases in the series Pt > Rh > Ir and Pd > Au, where platinum is considerably more active than palladium. It was found in [233] that the oxidation rate of propane in alkali decreases in the order Pd > Pt > Ag, and those of ethane ethylene, propylene, and n-butane decrease in the order Pt > Pd... 

---