S. B. BRUMMER AND M. J. TURNER
3902
The Adsorption and Oxidation of Hydrocarbons on Noble Metal Electrodes.
V.
Relation of “Reduced Carbon Dioxide”
to Adsorbed Hydrocarbons
by S. B. Brummer and M. J. Turner Tyco Laboratories, Inc., Waltham, Massachusetts
02164
(Received April 17, 1967)
The adsorption of a product-(‘reduced COZ”-on smooth Pt electrodes in COz-saturated solutions of 12 M H3P04at 130” has been studied by anodic stripping and cathodic H atom deposition. The amount of adsorbate is a maximum at low potentials (0 ‘v 0.8 at 0.2 v us. rhe) and declines to zero at -0.7 v. vs. rhe. Anodic desorption studies show that this absorbate has a constant structure in the range 0.15-0.35 v and may consist of two parts. The major part releases 1.2 electrons per site when oxidized back to COz and is the same as the 0-type adsorbed material reported previously for C3Hs and n-C&4. The minor part of the adsorbate may be equivalent to the CH-/3 product reported earlier for C3H8 and n-CaH14. It is shown that the 0-type material found with the hydrocarbons must have been produced directly from them and not from the reduction of COz evolved from their anodic oxidation.
I. Introduction This paper is part of a study of the basic mechanisms of the anodic oxidation of saturated hydrocarbons. Previously, we reported results for the oxidative adsorption of C3Hs1-3 and on Pt from solutions in 12 M H3P04 at 130”. An important result was the distinguishing of three generically different species in the steady-state adsorbate of the hydrocarbons. These species were of two main types: 0 type and CH types. The 0 type is predominant in terms of coverage at all potentials. It is the most highly oxidized of all of the species and the easiest to oxidize further (to COz) at high potentials. It was suggested that most of the C-H bonds of the original hydrocarbon had been converted to C-0 bonds in forming the 0 types (hence the name). The CH types, CH-/3 and CH-CY,were much less oxidized than the 0 type. They are harder to oxidize at high potentials (0.70 v) and probably represent a polymeric material and (partly dehydrogenated) alkyl radicals, respectively. Unlike 0 type, which appears to be a single adsorbed species, the composition of the CH types changes with formation potential and probably with hydrocarbon structure. The The Journal of Physical Chemistry
further purpose of our research program is to examine the properties of these materials and to attempt to define their roles in the over-all hydrocarbon-to-CO2 reaction. The 0-type species is of particular interest since the same material is found with various hydrocarbons3s4 and the coverage is always so high. It is clear that the path whereby it is formed is of great importance in defining the chemical mechanism of anodic oxidation. For these reasons, we have investigated the structure and reactions of 0-type adsorbed hydrocarbons in some detail. The present paper describes a comparison of the oxidation state of the 0-type adsorbed on smooth Pt at 130” with “reduced COZ.” The importance of “reduced COz” in the anodic (1) 5. B. Brummer, J. I. Ford, and hl. J. Turner, J . Phys. Chem., 6 9 , 3424 (1965). (2) 9. B. Brummer and M. J . Turner, “Hydrocarbon Fuel Cell Technology,” B. 8.Baker, Ed., Academic Press Inc., New York, N. Y ., 1965,
p 409. (3) S. B. Brummer and M. J. Turner, J . Phys. Chem., 71, 2826 (1967). (4) Part I V : 5. B. Brummer and ,M.J. Turner, ibid., 71, 3494 (1967). (5) 5. B. Brummer, J. Electrochem. SOC.,113, 1042 (1966).
ADSORPTION AND OXIDATION OF HYDROCARBONS ON NOBLEMETALELECTRODES
reactions of partially oxygenated fuels was first pointed out by Giner.s He investigated the products formed from methanol and formic acid solutions on platinized platinum electrodes and compared the anodic stripping curves for these materials with a product he had found adsorbed from Cos-saturated solutions. This product he had called “reduced C02.’” Subsequently,* he reported that a number of saturated hydrocarbons also give rise to “reduced COZ” at 95” in 2 N H2S0, solution. Niedrach and co-workersg have reported evidence similar to that of Giner for the presence of “reduced Con” on electrodes exposed to saturated hydrocarbons. Their studies were with Teflon-bonded platinum fuel cell anodes in HClO, and H3P04, working from 60 to 120”. Following Gilman,lo they distinguished two parts of their adsorbates on the basis of waves in their anodic stripping curves. The first of these, at less anodic potentials, they identified with a “CO-like” material similar to HCOOH,d, and “reduced COz.” Their evidence for this conclusion, like Giner’s, is based on the qualitative similarity of the peak positions for their so-called “type I” wave and the wave found in the presence of C02. The purpose of the present investigation was to examine the properties of reduced C02 using the anodic desorption method developed earlier.”.lz This method was used to characterize quantitatively the oxidation states of adsorbed C3Hs3and 7~-CaH14~ and therefore allows a quantitative and much less ambiguous comparison between the adsorbates.
11. Experimental Section Most of the details of the experimental procedure have been de~cribed.’-~ Experiments were carried out in purified SO% H3P04at 130” using annealed Pt microwire electrodes. COz was “Instrument grade” (Coleman, 99.99 vol. %) and was presaturated with water before being passed into the cell. Potentials are reported against the reversible hydrogen electrode in the same solution (rhe) although they were actually measured against the dynamic hydrogen reference electrode of Giner.13 Results are quoted “per real cm2,” based on 210 pcoulombs for cathodic galvanostatic H atom deposition on a clean e1ectr0de.l~ The adsorption and analysis procedure was as follows. The electrode was potentiostated a t 1.35 v for 2 min with gas stirring, to oxidize any adsorbed impurities. The last 30 see was without stirring if an experiment in quiescent solution was to be done. Oxide formed at 1.35 v was reduced a t the adsorption potential (0.15-0.35 v vs. rhe). After appropriate adsorption (usually 5 min with gas stirring, then 1 min without)
0
I 0.10
I 0.20
3903
I 0.30
I 040
I 0.50
a60
0.70
POTENTIAL ( V v s R.H.E.)
Figure 1. Charge to oxidize adsorbate as a function of potential at 130’: 0, 223 mm of COa; noncathodically desorbable residue under 223 mm of CsH8;la A, noncathodically desorbable residue under 102 mm of n-CBH14.
+,
the electrode potential was raised to a region where no adsorption normally occurs. Preliminary experiments (Figure 1) showed that at 0.75 v there was no adsorption from COrsaturated solutions and this potential was chosen. At 0.75 v, the adsorbate formed at the lower potential was gradually oxidized. The anodic desorption method consists of interrupting this gradual oxidation process to sample the residual absorbate. The adsorbate is sampled in each of two ways: either we oxidize that part of the adsorbate not yet oxidized (Q) or we measure the residual coverage of the electrode with the adsorbate (eorg). Q is measured with an anodic current (-50 ma/real om2), all the charge to O2 evolution being integrated and appropri(6) J. Giner, EZectrochim. Acta, 9 , 63 (1964). ( 7 ) J. Giner, ibid., 8, 857 (1963). (8) J. Giner, paper presented at 15th CITCE (Cornit6 Interna-
tionale de Thermodynamique et Cinetique Electrochimique) Meeting, London, 1964. (9) L. W. Niedrach, S. Gilman, and I. Weinstock, J. Electrochem. Soc., 112, 1161 (1965). (10) S.Gilman, Trans.Faraday Soc., 61,2546,2561(1965). (11) S. B. Brummer, J . Phya. Chem., 69, 562 (1965). (12) S. B. Brummer and J. I. Ford, ibid., 69, 1355 (1965). (13) J. Giner, J. Electrochem. Soc., 111, 376 (1964). (14) This has been discussed in some detail in ref 1-4 and 11.
Voupne 71, Number 1.9 November 1967
S. B. BRUMMER AND M.J. TURNER
3904
ate subtraction of the charge for electrode oxidation being made.3s15 The coverage, Oorg, is measured by plating H atoms onto the surface. This procedure gives e C H , the fraction of the maxinium H atom charge on a clean electrode. eorgis defined as 1 - e C H . l 6 Q is plotted against Oorg and changes in the slope of t,his relation are taken as indicative of changes in the structure of the original adsorbate. On the basis that the cathodic H atom charging method leads to a monolayer of H atoms,14 the slope of the Q us. Bo,, plot gives [e], the number of electrons released per covered surface atom when the adsorbate is completely oxidized to COz. This quant,ity is obviously of crucial importance in helping to define the chemical structure of the adsorbed species for these complex adsorbates. In addition, it provides the soundest and most unequivocal comparison between adsorbates found (with varying coverages) with different starting materials. 111. Results and Discussion
Coverage. The coverage of smooth Pt a t 130” with “reduced COZ”is shown in Figure 1. We see a typical coveragepotential relation found for an organic species. For comparison we also show the coverage with the noncathodically desorbable “residue” (=O type and CH-p) for C3Hs3and n-CsH14.~ Since the relationship between Q and e,, is almost linear (see later), the curve gives a good idea of the “reduced Cor’’ coverage. Maximum coverage a t low potentials is -0.85. At 0.4 v, the coverage is less than 0.25, and by 0.70 v it is zero. The shape of the curve reflects the increasing rate of reduction of COz a t low potentials and its increasing rate of oxidation at high potentials. Oxidation State of Reduced COz. The Q us. Oorg plots for reduced COz adsorbed a t various potentials are shown in Figure 2. The process of desorption occurs from the right to the left of the figure. That is to say that the material most easily oxidized for a given adsorbate, Le.! the first material to be oxidized at 0.75 v, is at the top right-hand corner of the figure. For adsorption a t saturation coverage (the 6-min data) at 0.25 v, Q vs. O initially (ie., high e) follows a line with a slope, [e], of 1.2 electrons per covered site. Coverages with this material are from
