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A. H. TAYLOR AND S. B. BRUMMER
impurities on alumina, but the decomposition cannot be ascribed t o the presence of peroxides or other impurities in alcohols. Efect of Oxygen. When the radicals formed by the photolysis of the surface complex were contacted with
oxygen, each of them changed to a peroxy radical, which showed the spectrum illustrated in Figure 10. The g values are g l = 2.003 and 911 = 2.032. The same spectrum was also observed when the irradiation was carried out under an oxygen atmosphere.
The Adsorption and Oxidation of Hydrocarbons on Noble Metal Electrodes. VII.
Oxidative Adsorption of Methane OM Platinum Electrodes’
by A. H. Taylor and S. B. Brummer Tyco Laboratories, Inc., Bear Hill, Waltham, Maasachusetts
05’164
(Received January 26, 1968)
The adsorption of CH4 on smooth and platinized Pt electrodes in 12 J4 HsP04 solutions at 130” has been studied with electrochemical pulse techniques. Anodic stripping provides a quantitative measure of the adsorbate. From 0.25 to 0.35 V (us. rhe), the adsorption rate in a quiescent solution is diffusion limited. At other potentials, mixed diffusion-activation control is operative. The low value of the calculated diffusion coefficient suggests that the initial adsorbate is rapidly oxidized further and indeed is oxygenated. Steady-state coverage is higher on smooth Pt than on platinized Pt and extends over a wider potential range. On smooth Pt, coverage is pressure independent below 0.30 V but increases with CH4 pressure at or above 0.35 V. Both the steadystate adsorbate and that sampled in the early stages of adsorption are similar t o previously reported 0-type hydrocarbons and to “reduced CO,;” i.e., they are oxygenated, C1 species. In no case were any cathodically desorbable hydrocarbon residues (CH-a) found on the electrode. The results show that C1 species are unstable and are rapidly oxygenated to form the 0 type.
Introduction It has been shown that fuel cells can be operated with saturated hydrocarbon anodes at economically interesting rates., Although the coulombic efficiency (with respect to C02) is 100%,3-sthe rates are 10w4r7despite stringent conditions of operation (85% at 150°227). A deep understanding of the mechanism of the reaction seems t o be required for the purpose of practical utilization. To this end, we have explored the reactions of C3H8,8-10n-C&4,’’ and C O P on platinum electrodes at elevated temperatures in concentrated H3P04; emphasis has been placed on the adsorbed species which accumulate on the electrode. have indicated that the reAdsorption action is extremely complicated, involving the formation of several adsorbed residues. These residues are of three generic types: CH-a, CH-p, and the 0 type. CH-a is cathodically desorbable, relatively unreactive toward oxidation, and probably comprises a mixture of (partly dehydrogenated) alkyl radicals.6 CH-P is unreactive toward both reduction and oxidation and is perhaps a carbonaceous polymer.lO The contribution of CH-a and CH-/3 increases with the molecular weight of the hydrocarbon.lo7” The 0 type, the major The Journal o j Physical ChernEstry
species in terms of coverage, is similar for C3Hs and n-C6HI4 and is apparently identical12 with “reduced C02.”13 The 0 type is the easiest species to oxidize at high p o t e n t i a l ~ . ~ g ~ ~However, -’~ there are sharp dis(1) Part VI: S. B. Brummer, Preprints, Division of Fuel Chemistry, 154th National Meeting of the American Chemical Society, Chicago, Ill., Sept 1967, Vol. 11, No. 3, p 178. (2) W. T. Grubb and L. W. Niedrach, J . Electrochem. SOC.,110, 1086 (1963). (3) W. T. Grubb and C. J. Michalski, ibid., 111, 1015 (1964). (4) H. Binder, A. Kohling, H. Krupp, K. Richter, and G. Sandstede, ibid., 112, 355 (1965). (5) E. Fleadi, et al., Unclassified Report No. AD632-319, The University of Pennsylvania, Philadelphia, Pa., April 1966. (6) G. Stoner, et al., Unclassified Report No. AD643-386, The University of Pennsylvania, Philadelphia, Pa., Oct 1966. (7) Report by General Electric Co. on Contract DA 44-009-ENG4909, Schenectady, N. Y., Dec 1963. (8) S.B. Brummer, J. I. Ford, and M.J. Turner, J. Phys. Chem., 69, 3424 (1965). (9) S. B. Brummer and M. J. Turner in “Hvdrocarbon Fuel Cell Technology,” B. S. Baker, Ed., Academic Press Inc., New York, N. Y., 1965, p 409. (10) S. B. Brummer and M. J. Turner, J . Phys. Chem.. 71, 2825 (1967). (11) S. B. Brummer and M. J. Turner, ibid., 71, 3494 (1967). (12) S. B. Brummer and M. J. Turner, ibid., 71, 3902 (1967). (13) J. Giner, Electrochim. Acta, 8, 857 (1963).
THEADSORPTION AND OXIDATION OF HYDROCARBONS ON NOBLEMETALELECTRODES agreements between the order of O-type coverage with respect to gas pressure (approximately zerolo,ll) and the order of the over-all reaction rate (approximately unity6j6). Mainly because of these disagreements, we postulated that formation of 0 type is very likely highly undesirable.lOvll Indeed, we felt that because this process consumes active C1 species involved in the favorable path of the over-all hydrocarbon oxidation, it may be the worst possible thing that can happen on the working an0de.l’ This conclusion may have to be modified, as we have found a negative order for C3H8 oxidation.’ However, we felt that an attempt to examine the processes leading to 0 type would be of considerable interest. A study of CHI adsorption seems ideally suited for this purpose, since we do not have problems with C-C bond breaking. For this reason, we have investigated CH, adsorption on Pt electrodes in H3P04.
Experimental Section The majority of the experimental procedures used are similar to those reported earlier.l0-l2 However, a number of modifications have been incorporated here. Water presaturation of the gases was controlled by a circulating liquid thermostat (k0.04”) instead of an air oven ( f l ” ) ,giving better control of fuel pressure. To reduce the possibility of atmospheric contamination and to control gas-flow rates more efficiently, Teflon needle valves were incorporated into the system. In place of the conventional three-compartment cell, one with a single compartment was used here. This cell contains two open-ended inner tubes within which are enclosed, respectively, the counter electrode and the autogenous Hz reference electrode developed by Giner.ls This setup reduces the possibility of electrolyte contamination and, in addition, is more easily handled. Matheson ultrapure grade CH, (99.95 or 9.95%, balance nitrogen) was passed through a Dry Ice surrounded cold trap before passage into the presaturator and cell. Both the N2 and CH4 gases used were freed from any dust content by passage through Millipore filters attached directly to the gas cylinders. Experiments were carried out in the hot (130”), concentrated (80%) H3P04 solutions. The H3P04electrolyte was purified by repeated recrystallization and H202 treatment as described previously. lo An additional purification by preelectrolysis was also carried out using a fuel-cell electrode. This was held in the solution for 48 hr at 0.40 V and 24 hr at 0.18 V with vigorous gas stirring. In practice after this treatment only 25 and 48 ,&/r cm2 of impurity charge (maximum) could be detected at 0.30 and 0.10 V (us. rhe), respectively, in solution stirred for 600 sec. The working electrode in most of these experiments was a smooth Pt plate (99.98%, Engelhard, roughness about 1.8) which was preannealed in an oxidizing flame.
2857
In some experiments a platinized electrode was used. It was prepared by reduction of a solution of HzPtCle (0.1 M in Pt, in the absence of lead acetate) onto a platinum plate previously roughened by immersion in aqua regia. It was thoroughly washed in distilled water, was anodized in dilute H2S04 to remove adsorbed chloride, and finally was aged for 24 hr in H3P04 at 130”. Adsorption measurements are related to the measured “geometric area” (geom cmz) and to the “real area” (r cmz). The latter was measured by applying a cathodic galvanostatic pulse and measuring the charge to deposit H atoms on the clean electrode ~urface.*,~0,~9,~0 A real square centimeter is assumed to be equivalent to 210 pC deposited in this manner,’O after correction for double-layer effects. This method was not possible directly with the platinized electrode (roughness about 100). I n this case, the area was estimated (f 10%) by reducing the oxide formed after 2 min at 1.25 V. Comparison with smooth Pt showed this oxide to correspond to 400 pC/r cmz. Potentials are reported against the reversible hydrogen electrode in the test solution (rhe).
Results and Discussion Anodic Stripping of CH,. As before,s-12 the anodic galvanostatic pulse method was used to estimate oxidizable adsorbed CH4. The potential-time curves during application of these stripping pulses were similar to those shown previously for C3H8.* As before, we must ensure that we appropriately correct for electrode oxidation and double-layer charging. For these purposes, all charge from the adsorption potential, Eads (but not including the H atom wave for low Eads),to 02 evolution was integratedz1 both in the presence and in the absence of adsorbate. Two charge quantities, QtotalCH4 and &totalN2, were examined as a function of measuring current density, i,. Results showed that this difference is independent of i, from 10-200 mA/ r cm2. This is similar to the findings for C3H8*and ?&6H14.” As then, it is taken to mean that during the transient all the adsorbate is oxidized to COz and there is perfect allowance for electrode oxidation. 2z Hence, the difference (&totalCH4 - &totalN2) equals QadsCH4, the charge to oxidize adsorbed CH4. Sub(14) s. Gilman, Trans. Faraday SOC.,61, 2546 (1965). (15) L. W. Niedrach, “Hydrocarbon Fuel Technology,” B. S. Baker, Ed., Academic Press Inc., New York, N. Y., 1965, p 377. (16) L. W. Niedrach, J . Electrochem. Soc., 113, 645 (1966). (17) L. W. Niedrach and M. Tochner, ibid., 114, 17 (1967). (18) J. Giner, ibid., 111, 376 (1964). (19) S. B. Brummer, J. Phys. Chem., 69, 562 (1965). (20) A. N. Frumkin, “Advances in Electrochemistry and Electrcchemical Engineering,” Vol. 3, P. J. Delahay, Ed., John Wiley and Sons, Inc., New York, N. Y., 1963. (21) Integration from the adsorption potential to Oa evolution
(complete stripping) has been shown to be the correct way to minimize errors from double-layer effects (S. B. Brummer, J. Phys. Chem., 7 1 , 2838 (1967)). Volume 72, Number 8 August 1968
2858
A. H. TAYLOR AND S. B. BRUMMER I
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sequent measurements were taken at about 100 mA/ r cm2, Adsorption Kinetics of CH4. The variation of &,deCH4 with time of adsorption and potential is shown in Figures 1 and 2. A potential step of 10 msec at 0.4 V was inserted before each measurement. This served to desorb any H atoms adsorbed on the surface at lower potentials and was shown not to affect &,dscH4. Initially, &,dsCH4 increases with 7ads1/’ independently of potential, for potentials from 0.25 to 0.35 V. Below 0.25 V, the rate of adsorption decreases with potential. It should be noted that over the entire potential range considered, the rate of adsorption of CH4 is apparently much slower than that found for CsH8ss9and n-C6H14.l1 The maximum adsorption rate from 0.25 to 0.35 V appears to be diffusionally controlled, since it is independent of Eadsand the adsorbate increases with 7ads1/’. This was verified by stirring the solution (Figures 3 and 4). Stirring greatly increases the adsorption rate at all potentials, indicating that under these conditions there is diffusional control initially at potentials from 0.25 to 0.45 V. Below 0.25 V there is mixed diffusion-activation control. The Journal of Physical Chemistry
Assuming semiinfinite linear diffusion, with no retarding effect on the adsorption due to blockage by adsorbed material, the accumulation of material on the electrode in quiescent solution is given byz8
For potentials between 0.25 and 0.35 V, the rate of accumulation of charge at the electrode was 33.7 k 1.8 pC/geom cm2/sec1”. The over-all oxidation reaction of CH4 to GOz is
CH4
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(2)
and n should, therefore, be 8. Solubility data are not available for CH,. If we assume that the solubility of CH4 is the same as for GHs, i.e., ~ 2 6 . 8X mol/ m I , 2 4 $ 2 5 then, from the data between 0.25 and 0.35 V, a value of 3.2 X 10-7 cm2/sec is obtained for DCH4 (22) Recently, we have shown (S. B. Brummer and K. Cahill, J . Electroanal. Chem., 16, 207 (1968)) that this nondependence on i, of the difference is not due to coincidental cancellation of errors but indeed results from correct allowance for the oxidation of the electrode. (23) H. A. Laitinen and I. M. Kolthoff, J. Amer. Chem. Soc., 61, 3344 (1939).
2859
THEADSORPTION AND OXIDATION OF HYDROCARBONS ON NOBLE METALELECTRODNS (130'). This is much lower than might be expected in comparison with the values of 7.9 X cm2/sec obtained for C3Hn10and 1.5 X 10-5 cm2/sec found for n-CoH14.l1 Coupled with the low measured rates of adsorption and the low steady-state anodic-stripping charges, this very low diffusion coefficient suggests tlhat after adsorption the CK, immediately oxidizes on the electrode surface. Subsequent galvanostatic sampling of the surface, then, reveals the presence of this oxidized species. If we take an extreme case, say, where the adsorbed hydrocarbon is actually CO, oxidation of this residue to CQ2 during anodic stripping would involve a two-electron transfer per adsorbed molecule. Substituting n = 2 in eq 1 leads t o a value for D C Hof~5.2 X 10-6 cm2/sec. This is a much more reasonable value for the diffusion coefficient, though it was not expected that the oxidation of adsorbed CH, t o a CO-like species would be so rapid. Figure 5 shows the change in the fraction of the surface available for H atom coverage, 8 ~ ( 1 3 0 ~with ), the adsorption time at 0.30 V. The purpose of measuring cathodic-charging curves is to examine the electrode surface for the presence of irreversibly adsorbed redidues. As for the anodic measurements, an additional potential step (0.40 V for 10m sec) was interposed in the potential-time sequence just before measuring a charging curve. We find that after an initial region of relatively slow adsorption (
