Electrochemical Oxidation of Certain Hydrocarbons and Carbon Monoxide in Dilute Sulfuric Acid

As a first stage in the investigation of the electrochemical oxidation of saturated hydrocarbons on platinum in acid electrolytes, the rate of adsorption of propane on a smooth platinum electrode was studied at 90~ in 37 mole % HF. Using an all-PTFE apparatus and single linear voltage sweep techniques, the surface coverage by hydrocarbon species was measured as a function of adsorption time and potential. The steady-state coverage resulting from propane adsorption shows a sharp maximum at 0.2v (vs. rhe). The amount of the most electrochemically active species is highest at 0.3v. The adsorption rate follows Langmuir kinetics with a third-order dependence on free surface. The rate constant has been measured for 0.2 and 0.3v at 90~During the last few years there has been an active interest in the direct anodic oxidation of saturated hydrocarbons in fuel cells (1-10). Exploratory fuel cell investigations pointed out the usefulness of strong acid electrolytes in this connection. The acidic electrolytes which have received the most attention in hydrocarbon fuel cells are ,sulfuric acid (1), phosphoric acid (2-4), hydrofluoric acid (5-8), and cesium fluoride-hydrofluoric acid mixtures (5,6,9,10). In order to understand better the kinetics and mechanisms involved in the over-all process of anodic oxidation of hydrocarbons, it is desirable to study the various individual steps in the process, starting with the adsorption of the hydrocarbon on a suitable electrode material in the presence of electrolyte.Some work on the adsorption of methane (11) and ethane (12, 13) on platinum in perchloric acid has been done at temperatures below those where significant reactivity is shown in fuel cells. The results of Gilman (12) on ethane adsorption on platinum in perchloric acid were interpreted in terms of the kinetics of the adsorption process, the adsorption being second order in free surface and first order in ethane concentration. Propane adsorption on platinum in phosphoric acid has been studied (14, 15), but since the adsorption process was diffusion-controlled, no rate expressions could be derived from those results. Butane adsorption on platinum in sulfuric acid has been measured at steady state by tracer methods (16), and under unsteady-state conditions by the use of flow-through porous electrodes (17); neither investigation yielded quantitative kinetic information.In order to study the kinetics of the adsorption process, it is necessary to perform the adsorption experiments under conditions where diffusion is not controlling. This requires that the product of the concentration of dissolved hydrocarbon and its diffusion coefficient be large enough (and therefore, diffusion fast enough) that the adsorption step is the slower process. It is known that hydrofluoric acid enhances the solubility of saturated hydrocarbons in water (18), and it is expected that the fluoride anion is not adsorbed on platinum electrodes (19,20). For these reasons and because of the fact that fuel cells using hydrofluoric acid electrolyte oxidize satu...

Section: Resultsmentioning

confidence: 99%

“…Since all of the terms on the r i g h t -h a n d side of Eq. [1] are small compared to QHc under appropriate experi- mental conditions, it is possible to obtain QHc to a high degree of accuracy, using small correction terms of only moderate (percentage) accuracy.…”

Section: Resultsmentioning

confidence: 99%

The Kinetics of Propane Adsorption on Platinum in Hydrofluoric Acid

Cairns

Breitenstein

1967

“…Although a significant research effort has been devoted in recent years to the determination of the mech-arAsm of electrooxidation of saturate hydrocarbon fuels, particularly those potentially useful in fuel cell systems, no clear picture of the rate limiting steps involved in such an oxidation is as yet available (1)(2)(3)(4)(5)(6)(7). While it is generally conceded that the adsorption or adsorption-cracking of saturate fuels on noble metal catalysts of the type generally used can constitute a significantly slow step in the over-all reaction, and has been suggested as the factor determining limiting current in some cases (5)(6)(7), no meaningful measurements of adsorption rates on practical catalysts, free of solubility and diffusion effects, have been made.…”

mentioning

confidence: 99%

Adsorption and Electrooxidation of Butane on Platinum Black in H[sub 2]SO[sub 4]

Shropshire

Horowitz

1966

Rates of butane adsorption on platinum black in sulfuric acid have been measured at 95~ using voltammetry in a flow-through electrode system. Initial adsorption rates showed a strong dependence on potential in the range 100-300 mv vs. the reversible hydrogen electrode. This effect was shown to ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.255.6.125 Downloaded on 2015-06-21 to IP Vol. 113, No. 5 ELECTROOXIDATION OF BUTANE 491

“…3-6 as well as the surface charge curves in Fig. The accumulation of these refractory species, which can be removed at still higher potentials, is also suspect as an important cause of the current and voltage oscillations frequently encountered with hydrocarbon fuel cells (10,11). The increase at 0.5-0.6v in going from methane to butane is of particular interest since it undoubtedly plays an important role in determining the magnitude of the maximum adsorption rate and hence the maximum current that can be drawn from an electrode, although other factors such as anion adsorption, surface oxidation, and bonding of water to the surface can also exert an influence.…”

Section: Discussionmentioning

confidence: 99%

Studies of Hydrocarbon Fuel Cell Anodes by the Multipulse Potentiodynamic Method

Niedrach¹,

Tochner

1967

The behavior of methane through butane fuels parallels that previously observed with methane, ethane, and propane with a perchloric acid electrolyte although quantitative differences are observed. A temperature range from 60 ~ to 120 ~ was covered. With both electrolytes all of the hydrocarbons form one class of surface species which is oxidized at relatively low potentials. Hydrocarbons above methane form a second more refractory class of surface species in both cases. As the molecular weight of the fuel increases, greater quantities of the refractory species accumulate on the electrode. These species play a role in determining the maximum current that can be carried by the electrode. They seem connected with the current and voltage oscillations frequently encountered with hydrocarbon fuel cells.