OXIDATIONAND ADSORPTION OF HYDROCARBONS ON NOBLEMETALELECTRODES
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Oxidation and Adsorption of Hydrocarbons on Noble Metal Electrodes. 111. CH-Type and 0-Type Intermediates during the Oxidative Adsorption of Propane on Platinum
by S. B. Brummer and M. J. Turner Tvco LabOratOTieS, Inc., Waltham, Massachusetts 02154 (Received JanUaTy 30, 1967)
The oxidative adsorption of C3Hson smooth Pt electrodes in 12 M H3P04solutions a t 110 and 130' has been studied. To ensure a clean reproducible electrode surface at each potential of interest, quantitative measurements of the adsorbate were made with anodic and cathodic galvanostatic pulses in conjunction with rapidly applied controlled-potential techniques. Adsorption occurs over the range 0.05-0.80 v (vs. rhe) with a maximum a t -0.20~. Theextent of adsorption increaseswith C3H8 pressure but less than proportionately. This steady-state adsorbate comprises several distinct parts, some of which are irreversibly adsorbed and chemically quite different from C3H8itself. The use of equilibrium isotherms to analyze the adsorption data is not then possible. Part of the adsorbate, the CH-a material, can be removed by cathodic displacement of the electrode potential and is most sensitive to C3H8pressure. Coverage with CH-a parallels the over-all adsorbate coverage in its potential dispersion (zero at 0.05 v, maximum at -0.20 v, and then a slom decline) and its positive dependence on C3H8pressure. Its composition is a function of potential and it is probably a mixture of almost unchanged alkyl radicals. The residue, that part of the adsorbate not cathodically desorbable, comprises two parts: the 0-type and CH-/3 materials. The 0 type is the major species on the electrode in terms of coverage and the most highly oxidized of all the adsorbed species, releasing 1.3 electrons per covered site on oxidation to COz. It has the same composition at all potentials and probably comprises oxygenated CI species. It oxidizes more readily than either of the CH types at high potentials. Its coverage is not sensitive to C3H8 pressure. CH-P is found below 0.30 v and is unreactive toward both oxidation and reduction. Its composition varies with potential. It is relatively unoxidized and is probably polymeric in character. The mechanisms of the production of these species and of the over-all C3H8-to-COzreaction are discussed. It is shown that the production of 0 type, while hitherto supposed to be favorable, is probably undesirable and a major limitation in the availability of the surface for the over-all reaction.
I. Introduction The present paper is the third in a series seeking to investigate the mechanism of the anodic oxidation Of saturated hy-drocarbons' This program was initiated following reports that fuel cells can be operated with saturated hydrocarbon fuels in concentrated Hap04 electrolyte a t elevated temperatures.' The need for a detailed understanding of the functioning
of the hydrocarbon anode follows from the observations that while the conversion of C3H8 (a relatively active ~ ? oxidation ~ kinetics are f ~ e l ~to, ~ ) is c o m ~ l e t e ,the
co,
(1) W. T. Grubb and L. W. Niedrach, J . Electrochem. Soc., 110, 1086 (1963). (2) Report by General Electric Co. on Contract D A 44-009-ENG4909, Dec 31, 1963. (3) H.Binder, A. Kohling, H. Krupp, K. Richter, and G. Sandstede, J . Electrochem. SOC., 112, 355 (1965).
Volume 71, ,Vumber 9
August 1967
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very ~ l u g g i s h . ~In the previous papers in this series, the oxidative adsorption of C3H8 on smooth Pt electrodes was studied at 80 and l10°5 and a t 130 and 140°.6 An important conclusion drawn was that during the course of its adsorption C3H8 is gradually oxidized to a partially oxygenated “residue.” The average oxidation state of this residue was not apparently dependent on potential (above 0.30 v us. rhe) or on the temperat ~ r e . ~This , ~ was interpreted5 as implying that the residue is a single chemical “species” over the whole range of observation. More sensitive experiments involving the oxidation characteristics of the residues (preliminary results of which were briefly reported earlier6) indicated, however, that there are at least three adsorbed “residues” involved in the steady-state adsorption of C3H8. The preliminary data suggested a significant anomaly in that the material found on the electrode at the more anodic potential (0.40 v) was less highly oxidized than that found at low potentials (0.30 v). If this tentative conclusion were to be maintained, it would suggest that the mechanism of the oxidation of C3HB t o CO, changes with potential (assuming of course that these residues are involved in the over-all C3H8-CO2 reaction). The purpose of the present paper is to pursue this apparent anomaly and to attempt to elucidate the number, the distribution-with-potential, and the character of the finally adsorbed residues. Subsequently,’ we will attempt to show in detail how the adsorbed species which we distinguish here are involved in the over-all reaction from C3?& to COz. The major result of this work is to demonstrate that in the steady state there are substantial amounts of a highly oxidized adsorbed species on the electrode (0 2 0.6). This species is the major constituent of the adsorbate at all potentials (0.22-0.40 v). I n addition, it is shown that there are a t least two other less highly oxidized materials on the electrode and that interference from one of these gives an experimental artifact which is responsible for the above-mentioned anomaly.
11. Experimental Section Since most of the experimental procedures have been described e a ~ d i e r ,only ~ , ~ the essential features and those involving changes will be indicated here. Experiments were carried out in a stirred oven (110 and 130’ (k0.5”))using a conventional three-compartment electrochemical cell constructed of Vycor. The gas lines t o and from the cell were of Pyrex glass and connections were made using cone and socket groundglass joints. These were sealed with Teflon sleeves and the joints were held tightly closed with ears and The Journal of Physical Chemistry
S. B. BRUMMER AND AI. J. TURNER
springs. Connections from the gas tanks to the glass were made with a short length of irradiated Teflon shrunk by heating. An important innovation, which significantly improves the length of the individual experiments, was to bring the separate gas outlets from each compartment of the cell to a central large bubbler. There they were all vented to the atmosphere by separate bubblers, but under the same triply distilled water. The significant gain of this procedure over the niore usual method using separate bubblers in separate containers follows from the fact that we transport a large quantity of water through the cell in the course of an experiment. In this way lye can be certain of raising the water level in the outlet bubbler by the same amount for each compartment. With separate bubblers we cannot be sure of this and we frequently generate a back pressure which pumps electrolyte out of one of the compartments of the cell. A number of experiments had to be terminated prematurely because of this. The electrolyte was ACS grade 85% H3P04(Lehigh Chemical Co.). This was filtered through glass and the hemihydrate (91.6 wt %, nip 29.3’9 was crystallized by cooling and adding a seed. After filtering, melting, and diluting with about 10% vol./vol. of triply distilled water, the hemihydrate was slowly recrystallized three times. The acid was then refluxed (as before,j except in Vycor) with 50% vol./vol. of “30%” H202 (Electronic grade, illlied Chemical) for 48 hr. A small quantity of the peroxide was added to the electrolyte in situ to destroy any impurities introduced during setting up the experiment. Tests with I < l I n 0 4 and titanium sulfate showed that all of the H202had been destroyed. Nz (“prepurified,” Matheson; passed through liquid N2 traps) or C3H8 (“instrument grade,” Matheson 99.5% minimum, or “research grade,” Philips Petroleum, 99.96 mole %--we have never found any difference for the present purposes between these two grades) was distributed to all three compartments of the cell as required. For the experiments at 130’, the gases were presaturated with water vapor by passing through mater a t 91”. This controls the mater vapor pressure at 537 mm,9the vapor pressure of 78 wt % H3P0410( ~ 1 2 M ) at 130”. (4) W. T. Grubb and C. J. llichalski, J . Electrochem. Soc., 111, 1015 (1964). (5) 5. B. Brummer, J. I. Ford, and 11. J. Turner, J . P h y s . Chem., 69,3424 (1965). (6) S. B. Brummer and M. J. Turner, “Hydrogen Fuel Cell Technology,” B. S. Baker, Ed., Academic Press Inc., New Tork, N. Y., 1965,p 409. (7) S. B. Brummer and XI. J. Turner, in preparation. (8) J. R. Van Waser, “Phosphorus and Its Compounds,” Interscience Publishers, Inc., New York, N. T., 1958, p 483.
OXIDATION AND ADSORPTION OF HYDROCARBONS ON NOBLEMETALELECTRODES
The cleanliness of this electrolyte was demonstrated in the following manner. A Pt microelectrode (see ref 5 and 6 for annealing procedure) was oxidized in the solution at 1.35 v us. rhe and then taken to 0.05 v t o reduce the oxide (10-1000 msec, with no apparent difference). It was then potentiostated at 0.30 v. At this potential, adventitious impurities in the solution can adsorb on the electrode (cf. ref 5 and 6) and will be found during subsequent oxidation and/or on subsequent H atom deposition. In practice, using the above procedures, less than 20 pcoulombs/real em2 of oxidizable material were found after 100 sec a t 0.30 v. Similarly, the surface was
