The Kinetics of Propane Adsorption on Platinum in Hydrofluoric Acid

Abstract

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...

The Kinetics of Adsorption, Surface Reaction, and Electrochemical Oxidation of Propane on Platinum in Hydrofluoric Acid

“…The first portion of this study showed that the rate of propane adsorption on platinum in hydrofluoric acid at 90~ is consistent with Langmuir kinetics, third order in free surface at both 0.2 and 0.3v vs. a reversible hydrogen electrode in the same electrolyte (rhe) (22). The formation of a highly electroactive species from the initially adsorbed "propane" was also reported (22).…”

“…1) in order to bring the electrode to a reproducible condition and prevent the accumulation of hydrogen or oxygen in the vicinity of the electrode. (22) and is similar to that used by Gilman (13) in related investigations. The potential was then stepped to 1.2v in order to maintain the oxide layer while the appropriate mass transport conditions were established (portion C of voltage sequence).…”

“…The single peak at 30~ is analogous to the type II peak at 90~ because of the similarity in peak voltages (1.3 at 90~ vs. 1.4 at 30~ (22) and because the peak at 30~ can be hydrogenation-desorbed as can the type II peak at 90~ (15,24). The type I peak at 90~…”

“…4, ref. (22) is necessary, but the single peak observed at 30~ is analogous to the type II peak at 90~ The main difference is that only one current peak is observed (near 1.4v) at 30~ while two were observed (near 0.Sv and near 1.2v) at 90~ 4 Because of this difference, no labeling of peaks (type I, type II, etc.)…”

“…The wiring diagram and additional details have been presented elsewhere (22). The wiring diagram and additional details have been presented elsewhere (22).…”